Cross-posted at Jennifer's History and Stuff.
Americans tend to think of black cats as bad luck, especially if one crosses their path, but the black cat is actually a very popular good-luck symbol in England today. Historically, cats in general have had a mixed bag of luckiness/unluckiness in Europe. In France, cats of any color that were suspected of being witches were caged and set on fire. In Eastern Europe, some cats were marked with crosses to prevent them from turning into witches.
Besides witches, black animals can be associated with other evil spirits. Demons tend to prefer to become black animals--whether cats, dogs, or other creatures. And it is believed that sorcerers often turn themselves into black cats.
The American superstition about black cats is based on the association of black cats with witches, originating in the late 1800s when cartoons and children’s books started pairing the two. Now you can hardly imagine a witch without her black cat keeping her company. Before this, witches in America were usually associated with rabbits and other animals.
Reference: Most of the material from this post was found in David Pickering's Dictionary of Superstitions and Steve Roud's The Penguin Guide to the Superstitions of Britain and Ireland.