Meet the siberian vampire moth. This little critter is believed to have evolved from moths which themselves would feed on the sweet contents of fruits, and like some other moth species, it's believed that this diet originated from the process of adaptation in order to feed on tears and other fluids from animals (also yes, there are moth species that pierce the skin of birds to feed on the salty contents of their lacrimal glands).
Essentially, apart from the necessary adaptations in the digestive track necessary to make the most out of a predominantly liquid diet, which many butterflies and moths already seem to do to an extent, all you need to achieve this is to make the tip of their long mouthparts, already adapted for drinking, into a more harpoon-like structure capable of piercing the skin.
So overall the presence of real life examples and a lifestyle already mostly adapted to a similar type of diet makes it pretty likely that your butterflies could feed on blood.
Now, let's talk about the problems associated with a hematophagus diet:
Nutritional value. Surprisingly enough, blood is actually not all that good as a food source. Human blood for example is 78% water, and out of the actually nutritious parts you'll find it to be composed of about 94% proteins, 1% sugars (carbohydrates) and almost zero vitamins and minerals. Vampire bats have adapted to this diet by having very high diversity of gut bacteria which is shares a symbiotic relationship with, making the most of its blood meals. Something else noteworthy about vampire bats, despite not being arthropods is that their need to feed often still indicates the problems of this diet: a vampire bat will starve if it goes more than 3 days without a decent blood meal. Your giant butterflies, despite not necessarily having a metabolism as high as a similarly sized mammal's, will most likely Also need to feed often in order to survive long enough to find a mate and lay its eggs in a suitable place.
prey. A very common trait among virtually all creatures which feed on blood is that they're smaller than whatever they're attacking. All blood-eating creatures are, in a way or another, parasites, and one key trait of all parasites is that a dead host is no good for them. The only potential counter example to this would be vampire spiders, which don't feed exclusively on blood but prefer to target female mosquitoes which have fed on human blood. Another advantage of being a parasite much smaller than what the thing whose blood you're sucking is that so long as you don't make a big fuss or cause pain, you might be able to go unnoticed, such as how vampire bats silently creep on the large mammals they feed on, as this ensures they only need to be strong enough to anchor themselves while feeding, rather than having to, say, pin down a mouse during the entire meal. Not only could that potentially kill the rat, it'd demand much more energy.
So in other words, according to what we normally see on earth, your vampire moths/butterflies would likely not act as predators per say, but rather as parasites of much larger creatures.
Hydration and impurities. As I said before, blood is mostly composed of water, but it's also composed of things such as ammonia and urea, meaning that in addition to the already present methods of easily getting rid of the excess water many butterfly species have due to their natural diet, your butterflies should be able to handle and more specifically get rid of this extra amount of additional toxins. This should be the easiest thing to solve out of all of these topics, and chances are that they could manage this without a problem.
diseases. Another thing vampire bats need to engage in their blood drinking activities is a strong immune system due to the risks of the blood they feed on carrying blood-borne pathogens that could make them sick and even kill them. Your butterflies too might benefit from a sturdy immune system to prevent itself from accidentally being infected by blood-borne pathogens in your world. Again though, the presence of vampire moths show that this isn't that major.
So essentially, it's very likely that your butterflies could have a blood-based diet without problems, because we have real life confirmation that it's possible. The big wrench in the gears here is that, overall, a mostly blood-based diet is incompatible with a predatory lifestyle, with most of if not all animals that feed exclusively on blood acting as parasites to larger, normally warm-blooded animals.
"but I want them to be predators and have a liquid diet, what do I do?"
Then you need to look at a different strategy: instead of simply adapting to feed on blood, your butterflies would need to adapt to a different type of liquid diet: one more like the Japanese water bug's.
Japanese water bugs are also creatures adapted for a more liquid diet, not because they simply drink the blood or tears from whatever they catch, but because they use their sturdy dagger-like mouthparts to stab their prey, injecting enzymes and anesthetics that break whatever they catch down into a nutritious liquid which they drink through the very same proboscis they used to inject said enzymes. These bugs also rely on powerful raptorial front legs (essentially front legs adapted for grabbing things, a good example is the praying mantis'forelegs) to pin down prey while they let the enzymes they injected do the rest. Their size and potent digestive chemicals allow them to prey on a variety of creatures, including snakes, fish, baby turtles and even ducklings.
In order for your butterflies to have such a diet, they'd need to undergo some changes. I do believe a moth-like body plan would be more suitable as their habit of keeping their wings more well tucked to their bodies would make it less likely that they'd be damaged. As for the other anatomical changes they'd require to be predators: not only would they need to change their long flexible mouthparts for sturdier shorter ones, more in line with what's seen on water bugs, assassin bugs and other insects that engage in such a predatory behavior, they'd need to adapt their legs into raptorial appendages suitable to pinning down prey. My personal guess is that their front legs would resemble those of an ochteran mantis fly in many ways (see below).
So summing up: can they feed on blood? Yes, but that'd make them parasites and would likely leave them dependent on the presence of large enough creatures for them to feed on. Could they be predators? Also not impossible, but rather than just blood they'd drink the liquefied remains of whatever they managed to catch, and would most certainly require changes to their mouth parts (particularly involving them becoming shorter and sturdier in addition to the adaptations related to piercing), the ability to produce and secrete special digestive enzymes, their legs to adapt into stronger raptorial limbs more in line with a predatory lifestyle centered around grabbing hold of and pinning prey down while the digestive enzymes to the magic and potentially their wings would need to be capable of folding more tightly against their bodies in order to minimize the risk of damage by the wild flailing of caught prey trying to escape.