From Longman Dictionary of Contemporary Englishblack sheepˌblack ˈsheep noun [countable usually singular] BAD PERSONsomeone who is regarded by other members of their family or group as a failure or embarrassment Amy’s always been the black sheep of the family.
Examples from the Corpus
black sheep• Are twins to become the moral equivalent of black sheep?• Then I became the black sheep and I found to my horror that everyone thought I was off my bloody head.• Even in this questionable industry, meanwhile, the Connecticut leasing operation is clearly the black sheep.• Faith is treated as the odd man out, the misfit, the black sheep.the black sheep of the family• I was always considered the black sheep of the family, the weirdo in the group, right? 