We are developing a game setting and, while discussing certain aspects of the it, have stumbled upon our inability to adequately account for some phenomena due to our lack of knowledge. The thing is, in wake of a catastrophe the lower layers of the world’s atmosphere are filled with a dark almost opaque substance, thus an expansive region of the planet hardly gets any sunlight. We are uncertain to what extent such a hindrance to natural light would cool the surface and whether it is conceivably possible for a substance to only block the visible spectrum, while still allowing some portion of the heat to come through (as infrared rays or whatever it is that normally transmits it). We would like to arrive at a bearable science-ish explanation, so your comments on the relevant physical, environmental and chemical processes are welcome.
Basically, we want for climate to stay relatively the same, somehow, even with constant dusk.
I think we may have asked the question in a slightly misleading way. The world we're trying to describe is not a 100% alternative Earth, as many of the suggestions seem to assume, but more of a fantasy/steampunk alternative world. So many of the problems may be solved by saying "because magic". On the other hand, we wanted to have enough understanding of the science behind the relevant natural processes so that we could plausibly explain how the population desperately attempted to at least partially fix the effects of the calamity by magical means (e.g., based on your suggestions, magically modifying wheat and some other plants, which turned dark grey/black in order to be able to absorb the remnants of the sunlight; red tint of the light coming through the substance in the sky also seems appealing for providing a more gothic undertone).
The substance blocking the sunlight is basically a result of a failed magical experiment and, as we plan it to be right now, mostly immaterial (basically, as we understand based on your suggestions, it should block 80-90% of all light sans the orange and red spectrum and above, so that the plants can somehow still photosynthesize after having been tampered with).
In the end, we want the world to be a dark and a slightly gothic one, but at the same time with populace more on the optimistic and pragmatic side (even if in the first, say 10-100 years after the calamity there were many losses and conflicts), which has found ways to influence their surrounding's bad conditions (adaptation of the flora; possibly coping with drops in temperature?). Basically, we're not very fond of desperation and decadence.
Also, the substance in question will cover approximately half of the planet, gradually thinning as we move away from the epicenter.
And the world is supposed to have two suns.