According to superstition, having a black cat cross your path may be lucky or unlucky, depending on where you live.
In some countries, such as Egypt, all cats no matter what their color are lucky. In Egypt, all cats were once considered so sacred that if one was accidentally killed, its killer was put to death. Cat cemeteries were common, and mummified cats were buried with their owners.
Black cats enjoyed no such exalted status during the Middle Ages, when it was believed that a witch could take the form of a black cat. A story about an English father and son meeting a black cat they suspected of being a witch proves the point. They stoned the cat and the next day encountered the witch in her human form. Her face was bandaged and she died shortly thereafter.
This story seems to be the origin of bad luck coming your way if a black cat crosses your path.