A person who is considered a disgrace to a particular group, usually a family

Every family has a black sheep, sooner or later

Ogni famiglia, prima o poi, ha la sua pecora nera

My grand-grand-grandfather is the black sheep of the family: he was a pirate

Il mio trisnonno è la pecora nera della famiglia: era un pirata

Black sheep were traditionally disliked by shepherds, as they were less valuable than white ones, because their wool could not be readily dyed. By the late 18th century, it had come to mean a person out of favor, someone oddly different and therefore a renegade