Familiars

In the Middle Ages in Europe, black cats became known as familiars and were associated with witches

They were then deemed bad luck and the stigma began.

Fable of Familars

There are theories as to why the black cat was correlated to witches. One folktale suggested that a father and son were walking home one evening and the boy threw a rock at a black cat crossing the street. It injured the cat's left leg and it limped under the house of a woman who was long suspected of being a witch. The next day the father and son saw the witch at the marketplace limping on her left leg. Since then, a fable formed that witches could shapeshift into black cats.

Religion

After the fall of the Roman Empire, the Catholic Church decided to take matters such as heresy seriously. This meant witchcraft was under investigation and trial. The Pope ended up issuing the Papal Bull which connected black cats to the devil in some shape or form. This drove fear through the people and led to the mass killing of black cats. Eventually in 1484 black cats were prosecuted with their witch owners by the Church.