Back when northern Europeans were being forced at the point of the sword and flame to convert to Christianity, the Germanic and Scandinavian peoples worshiped Freyja (goddess of love, sex and magic). Cats were (and still are) Her sacred totem animals. When Christianity took over, Freyja was vilified as a demon by the Church and her totem animals degraded as minions of wickedness. Church officials encouraged the killing of cats by the "righteous", and fabricated horrific tales about cats to frighten parishioners from ever thinking there was anything benign about a pagan goddess. As the color black has long been associated with death among Christians, many of the faithful came to believe black cats are particularly evil. Hence also the stereotype of the "ugly witch" (a deliberate misrepresentation of a follower of Freyja), with her warty nose, cannibalistic hunger for children and, of course, her black cat familiar. As time passed, the superstitious came to fear the black cat so much they believed that one crossing their path had the ability to curse them, and so they thought by making the sign of the crucifix or cross they would be protected.
Incidentally, the number 13 is one of Freyja's sacred numbers, which also helped stigmatize it as an ill-omened number in the eyes of the superstitious.