Black sheep have a bad reputation when saying things like, "they are the black sheep of the family." And black crows also have superstitions about them, same with ravens, but that may have to do with The Raven by Edgar Allen Poe. I am guessing, but it is probably only that. I do know that pure black cats are sometimes prized by those who are not afraid of black cats. They call them void kitties, and if they have yellow eyes, that is even better. I think at night, if a pure black cat would be on the street, one might only see the glow of their eyes. That might play into why they were thought to be related to witches or have an evil spirit inside of them or something. I find it creepy but fascinating to see it myself.