Part One of Two: A More Flight Friendly Planet.
When it come to questions of habitability, who are you gonna check?
There are many scientific discussions about the habitablity of planets for carbon based, liquid water using lifeforms, with basic biochemistry similar to that of Earth lifeforms. But there is only one scientific discussion that I know of about the more restricted problem of the habitability of planets for human beings, or for large bodied multi celled oxygen breathing animals having similar environmental requirements to human beings.
Habitable Planets for Man, Stephen H. Dole, 1964, 2007.
Any writer who ever expects or desires to write a story set on a planet habitable for humans, or habitable for lifeforms with similar requirements, should study it.
In Chapter 4 The Astronomical Parameters, section Planetary properties, Dole discusses the possible mass range of habitable planets. On page 53 he states that the planet should have a surface gravity of less than 1.5 g, corresponding to a mass of 2.35 earth, and a radius of 1.25 Earth radius (7,988.75 kilometers), and an escape velocity of 15.3 kilometers per second. Note such a planet would have 1.5 times the surface gravity of Earth and 1.36 times the escape velocity.
Dole says that the planet should retain oxygen in its atmosphere for geologic eras of time. To do that it should have an escape velocity of at least about five times the root-mean-square velocity of atomic oxygen in the exosphere of the planet. So the escape velocity of the planet could be as low as 6.25 kilometers per second, 0.558 that of Earth, and the planet would have a mass of 0.195 Earth, a radius of 0.63 Earth (4,013.73 kilometers), and a surface gravity of 0.49 g. It would have 0.49 Earth's surface gravity and 0.558 Earth's escape velocity.
But Dole believe such a planet would be too small to produce a dense, oxygen rich atmosphere. In the next few pages Dole estimated two separate minimum masses for a planet which could produce a dense, oxygen rich atmosphere, 0.25 Earth mass and 0.57 Earth mass. And Dole decided that the true value would be somewhere between them, somewhere about 0.4 Earth mass, corresponding to a planet with a radius of 0.78 Earth radius (4,969.38 kilometers) and a surface gravity of 0.68 g. According to my rough calculations such a planet would have a escape velocity of about 8.01 meters per second, or about 0.71 that of Earth.
What is necessary to make flying as easy as possible on a fictional planet is the somewhat contradictory requirement to have as low a surface gravity as possible and as dense an atmosphere as possible. And note that the ability of a planet to retain atmosphere is dependent on its escape velocity, not its surface gravity. The formulas for calculating surface gravity and escape velocity are different.
Note that the more massive an Earth like planet is, the more its gravity will compress its materials and increase the average density of the planet. As a result, Dole's example of a planet more massive than Earth has 1.5 times the surface gravity of Earth and 1.36 times the escape velocity.
The ratio of surface gravity to escape velocity is reversed in Dole's two examples of planets with less mass than Earth. A planet with about 0.4 Earth mass would have about 0.68 Earth's surface gravity and 0.71 Earth's escape velocity. A planet with about 0.195 Earth mass would have about 0.49 Earth's surface gavity and 0.558 Earth's escape velocity.
Thus a writer desiring to make flying as easy as possible on a habitable planet should choose a planet with a lower mass than Earth, so that the escape velocity to retain an atmosphere will be higher relative to the surface gravity.
And maybe the smallest planet that could produce an oxygen rich atmosphere might be smaller than Dole estimated. It might possibly even be as small as the smallest planet that could retain an oxygen rich atmosphere, with a mass as low as 0.195 Earth, a surface gravity of 0.49 Earth, and an escape velocity of 0.558 Eerth.
Or maybe advanced aliens terraformed that small planet, producing a dense and oxygen rich atmosphere, some time in the past. I note that the planet's atmosphere is desired to be dense and opaque enough to block most sunlight from reaching the surface, and thus there shouldn't be photosynthesis to produce an oxygen atmosphere, so either some little understood natural process, or photosynthesis by tiny organisms floating high in the atmopshere, or terraforming by an advanced civilization in the past, would seem to be necessary to produce the oxygen that large flying lifeforms would probably need.
You might even possibly have a dense oxygen nitrogen atmosphere on a planet that is smaller than Dole's minimum size to retain an oxygen atmosphere. That small planet would have to orbit outside what is usually considered the habitable zone of its star. The amount of light it gets from its star is insufficient to keep the surface warm enough for life, and so the root-mean-square velocity of atomic oxygen in the outer layer of the atmosphere will also be much lower than Earth's, enabling the planet to retain its oxygen.
The planet's surface would need to be warmed by internal heat. Possibly the planet is actually a giant exomoon of a giant exoplanet and tidal heating produces the internal heat necessary for life. And the oxygen in the atmosphere would presumably have been artifically produced by terraforming by an advanced civilization.
If flying is several times as easy on that planet as on Earth, due to lower gravity and a denser atmosphere, the largest possible flying creatures on that planet should be several times as large as the largest flying creatures in Earth's history. I can't help thinking that the largest possible flying creatures on that world would be real "terror dactyls".
Part Two of TWo: Bird Brains.
Maybe it wouldn't be necessary to have an atmosphere much denser than Earth's to have flying beings large enough to be as inelligent as humans. The OP wants a dense atmosphere to make flying easier for large beings and to make the surface dark so the natives are blind and use ecolocation instead.
But even the Venerean atmosphere isn't thick and opaque enough to make the surface pitch black.
If the light level at the Venerean surface is comprable to an overcast day on Earth, Earth lifeforms can see fine in such light levels and even much lower light levels.
So instead of making the atmosphere super dense to block all light from the surface, possibly the atmosphere could have a density similar to that of Earth, but with varius layers of dust or chemeicals high in the atmosphere that block various wavelengths of light, just as the ozone layer blocks most wavelengths of ultraviolet light. Enough such layers might render the surface and the lower atmosphere totally dark.
So even an atmosphere as dense that that of Venus is not opaque enugh to make vision impossible at the surface, and an atmosphere only as dense as Earth's could conceivably have layers of dust or chemicals which might make vision impossible at the surface.
Could beings large enough to be intelligent be able to fly in an atmosphere no denser than Earth's?
There are thousands of species of small mammals on Earth, and hundreds of species of large mammals. Any large mammal species could possibly support a large brain, and about a hundred species of large mammals do support large brains, roughly on the order of size of the human brain. So those hundred or so species of large brained primates, proboscideans, and cetaceans could conceivably have intelligence ranges significantly overlapping the intellligence range of humans.
Thus it is possible that unbaised extraterrestrial observers might decide that one or more or possibly all of those hundred species should be considered to be, like Homo sapiens, partially or even fully intelligent.
Of course none of those species fly naturally.
An adult human will typically have an average body mass of about 57.7 kilograms (127.2 pounds) to 80.7 kilograms (177.9 pounds) depending on where they live.
At the present time the largest flying bird species is the mute swan with an average mass of 11.87 kilograms (26.2 pounds) and a maximum mass of 23 kilograms (51 pounds), about 0.147 to 0.398 of the mass of an adult human.
The largest extinct flying bird Argentavis magnificens, had a wingspan of 5.09 to 6.5 meters or 16.66 to 21.33 feet, and an estimated mass of 70 to 72 kilograms (154 to 159 pounds). That mass would be about 0.867 to 1.247 times the mass of an average adult human.
One of the largest extinct flying reptiles known, Quetzalcoatlus northropi, had a wingspan of 10 to 11 meters (33 to 36 feet).
Weight estimates for giant azhdarchids are extremely problematic because no existing species share a similar size or body plan, and in consequence, published results vary widely.2 Generalized weight, based on some studies that have historically found extremely low weight estimates for Quetzalcoatlus, was as low as 70 kg (150 lb) for a 10 m (32 ft 10 in) individual. A majority of estimates published since the 2000s have been substantially higher, around 200–250 kg (440–550 lb).
So if Quetzalcoatlus northropi had a mass of about 70 to 250 kilograms, (150 to 550 pounds), it owuld have a mass of about 0.867 to 4.332 that of an average adult human.
Thus it seems theoretically possible for flying beings as large as the largest known extinct flying beings on Earth to be able to support brains as large as human brains, and thus to achieve human intelligence levels.
An atmosphere several times as dense as Earth's, and lower gravity than Earth's, are not absolutely necessary for flying beings to have human sized brains and human intelligence levels.
But there's more!
Some groups of birds, such as crows and parrots, display anomolously high levels of intelligence, considering that that their entire bodies only wigh a few kilograms or pounds, and their brains weigh only tiny fractions of their total body weight.
Thus it could be hypothosized that because flying creatures like birds need to minimize weight, at least some birds have evolved highly efficient brains that work much more efficiently than mammal brains. So it is conceivable that a bird brain a fraction the size of a human brain could produce human levels of intelligence.
And the pressure to maximize the weight efficiency of organs, including brains, of flying creatures might also operate on an alien planet. Thus some flying creatures on alien planets might be far more intelligent than the size of their brains relative to Earth mammal brains would indicate, and intelligent flying beings might be be significantly smaller than humans in total mass and in brain mass.
This might make it easier for flying beings to attain human intelligence levels on a planet similar to Earth, and maybe even easier to do so on a planet where conditions make flying much easier than on Earth.