Basically - and with most things - the beliefs surrounding black cats is all based down to cultural beliefs. Some places see them as lucky and some unlucky.
Here are some of the beliefs I came across: -
Most common belief is that they are witches familiars.
|MJ and Phantom|
*Looks around for her many suitors... "I don't seem 'em and I've got two cats. Shouldn't I be rich?"*