I can has ur soul plz?
It didn't help that a number of pre-Christian peoples such as the Norse, Celts and Egyptians had cat gods, or at least considered the animal sacred. Once Christianity became the sexy new religion in town, old beliefs were branded witchcraft and cats found themselves guilty by association. Often simply owning a cat was considered proof of witchery. There was even widespread extermination of cats during medieval times, which kind of backfired when they were no longer around to kill plague-infested rats, which in turn wiped out half of Europe. Whoops. But hey, at least they were safe from those goddamn witches.
As for why black cats specifically were feared, well, you don't need us to tell you that black has traditionally been associated with eeeevil. There's a reason Darth Vader didn't spend his time strutting around in a sporty magenta or mint green get-up.