A black cat is just what the name suggests, a cat who has black fur. It does not have to be any particular breed, it can be purebred or mixed. The black color is a help to cats that hunt at night.
A look into history shows that cats that have all-black fur were considered to be symbols of evil in many places. However, there are also a few cultures where they were omens of good luck. Strangely enough, these superstitions have never really gone away even in modern times. It is a fact that there is a lower adoption rate for black cats in American shelters, than those of other colors.
Back in the day, these cats were strongly associated with evil and witchcraft. In Hebrew and Babylonian folklore, they were symbolic of serpents coiled on a hearth. In ancient Egypt, however, they were worshiped. When the family cat died, the whole family would go into mourning and the cat's body would be mummified. This also continued with the Romans, who introduced this cat to Europe. Being greatly worshiped, black cats were considered as a sign of good luck at that time, especially if one crossed your path. This worship was attributed to the Egyptian cat goddess Bastet. Egyptian families believed that they could remain in her favor if they worshiped black cats.
However, around the 17th century, the church drastically changed this opinion and began to associate these cats with witchcraft. Due to this, black and even white cats began to get a bad name. Instead of being something that was worshiped, they began to be feared. The countries where they suffered the most were those that held witch hunts.
However, there were a few countries that still considered them to be a god sign. In Scotland, a strange black cat on your porch was the sign of prosperity soon to come; fisherman's women kept them believing that it prevented anything bad from happening to their loved one while he was at sea. However, in Ireland, a black cat crossing your path in the moonlight meant that illness was going to come to you. In Romania, Moldavia, and even the Czech Republic, this superstition is still alive today. This belief once had a very strong hold on the American culture, but not so much today. They once believed that a black cat crossing your path was bad luck, but it brought good luck if you let one into your house.
The 'Black Cat in a Fighting Stance' was adopted as an anarchist symbol, more specifically for a branch of anarchism that focused on workers rights.
There is also a legend about these cats and sports. A team was winning a game when a black cat came in and circled around a third baseman. The team lost that game miserably. Back in the beginning of America, black cats were thought to be the friends or familiars of witches. Those who practiced witchcraft were believed to use these cats to help participate in rituals, sometimes as a sacrifice. In the trials that took place, a witch's ownership of the cat was taken as evidence of Satanic association. Some of these cats were even burned alive with their owners during the trials. It is thought that even today, there are groups that engage in the ritualistic sacrificing of cats. Some think that is urban legend, while some believe that it really happens.
So are they evil or are they good? Depending on what country you go to and ask that question, you will get different answers. One, that they are the most evil of creatures and are to be avoided, and the other that they are just animals and should be treated no differently then any other cat. It depends on every individual person.