black sheep (n.)

by 1822 in figurative sense of "member of some group guilty of offensive conduct and unlike the other members," supposedly because a real black sheep had wool that could not be dyed and thus was worth less. But one black sheep in a flock was considered good luck by shepherds in Sussex, Somerset, Kent, Derbyshire. First known publication of Baa Baa Black Sheep nursery rhyme is in "Tommy Thumb's Pretty Song Book" (c. 1744).