A black knight is a soldier or knight who either is not in service to a certain liege or does not want his liege, or himself, to be known and so does not have any heraldic standards or has painted them over with black paint. A good and well equipped soldier without a certain fealty is a wild card, or someone with the freedom to act who could be surprising. A group of them could be a big problem for kings. Usually, a black knight is put in a bad light, as a soldier who has covered up his standards so it could not be found out where they came from.
In made up stories, black knights are usually evil knights that are good at fighting and usually work alone. A good example of this would be the Arthurian legend of the black knight.