People will say that black cats go unadopted more often anecdotally, but from articles on the subject, it seems to be not necessarily the case that black cats are less wanted than other colors of cats.
Weiss found that, although euthanasia numbers for black animals are at
or near the top (both black and white dogs were near 19 percent; black
cats were at 30 percent, with gray cats and white cats coming in just
under that, at 28 percent and 26 percent, respectively), their total
adoption numbers were also the highest of any color.
Black is simply a common color for cats, probably in part because it helps camouflage them from predators in the case that they are a stray or feral cat. As a result, people observe that there are many more black cats not being adopted, and don't consider that they are just plain more of them, it seems.
The idea of black cats being less wanted is also something of an urban myth. People will very, very commonly repeat this information without stopping to check whether it is true or not, probably in part because they've heard it said so often.
People in western countries also are commonly aware of the superstition that a black cat will bring bad luck, which most people assume is the reason for the claim that black cats are not wanted as pets. However, despite the awareness of the superstition, nearly everyone in Western countries do not actually believe it. I think people would find anyone who said they actually believed it to be very odd.
I think if there is any truth to black cats being less wanted, it's because black cats are common and, though not considered ugly, it's not particularly striking compared to certain other coat colors or patterns that a cat might have either. That is to say, a black cat might seem boring compared to another unique looking cat.