black sheep n.

by 1792, in the figurative sense of "person of bad character; member of some group guilty of offensive conduct that does little credit to the flock, family, or community to which he belongs," supposedly because a real black sheep (there was proverbially one in every flock) had wool that could not be dyed and thus was of less worth. But one black sheep in a flock is said to be considered good luck by shepherds in Sussex, Somerset, Kent, and Derbyshire. The first known publication of Baa Baa Black Sheep nursery rhyme is in "Tommy Thumb's Pretty Song Book" (c. 1744).