Depending in where you come from in the world having a black cat cross your path can either be bad luck or good luck.
Traditionally black cats are associated with Witches and evil, which often gives them the stigma of being bad luck.
However, luck is what you make it. If a black cat crosses your path, and you are so busy looking at the cat you do no do see the curb stone at the edge of the road, and you fall over it injuring your arm, then you are most likely to consider a black cat bad luck. On the other hand, if you have a black cat cross your path, and next to it is a ten dollar bill that some one has dropped and you pick it up, then you will consider the back cat to be good luck.
What ever your first experience was with a black cat, which will make the decision as to whether the black cat is good or bad luck for you.
Pure black cats are actually quite a rare thing. Often they will have little specks of other colors in their coats or on their paws. And very often if there a pure back cat has been bred with a tabby cat at some time in the cats ancestor the cat will appear to be black, however when the light is right you will be able to see the tabby markings very faintly on the coat.
The true black cat is the Bombay cat, which is an Asian cat and is completely black. This cat originates from Burmese cats and has a pure black coat, paws and nose with copper and green eyes. The Bombay black cat of Asian origin should not be confused with the American cat of the same name, which is a newer breed of cat originating in 1958.
So are black cats good or bad luck? They could be either. Next time you see a black cat, make sure you are alert so nothing bad will happen and perhaps something good will instead!