Superstition, Prejudice, Bringer of Good or Bad Luck...
The folklore surrounding black cats varies from culture to culture. In Great Britain, black cats are a symbol of good luck. The Scottish believe that a strange black cat's arrival to the home signifies prosperity. Furthermore, it is believed that a lady who owns a black cat will have many suitors. However in Western history, black cats have often been looked upon as a symbol of evil omens, specifically being suspected of being the familiars of witches; other cultures also consider them to be bad luck. (Sometimes, other black creatures, such as black dogs, also shared in the prejudice and suspicion of being "familiars").
The black cat in folklore has been thought to change into human shape to act as a spy or courier for witches or demons. During the Middle Ages, these superstitions led people to kill black cats. This had the unintended consequence of increasing the rat population and the spread of the black plague and other diseases carried by rodents. There is no evidence from England of regular large-scale massacres of "satanic" cats, or of burning them in Midsummer bonfires, as sometimes occurred in Europe.
However, the supernatural powers ascribed to black cats were sometimes viewed positively, for example by sailors considering a Ship's cat in general and a black one in particular as lucky for their ship. Sometimes, fisherman's wives would keep black cats at home too, in the hope that they would be able to use their influence to protect their husbands at sea.
Black cats have been found to have lower odds of adoption in American shelters compared to other colors.