It was largely in the Middle Ages that the black cat became affiliated with evil. Because cats are nocturnal and roam at night, they were believed to be supernatural servants of witches, or even witches themselves. Partly because of the cat's sleek movements and eyes that 'glow' at night, they became the embodiment of darkness, mystery, and evil, possessing frightening powers.
If a black cat walked into the room of an ill person, and the person later died, it was blamed on the cat's supernatural powers. If a black cat crossed a person's path without harming them, this indicated that the person was then protected by the devil.