the black sheep ( an idioms) the black sheep (of the family) someone who is thought to be a bad person by the rest of their family My father was the black sheep - he ran away at 16 to become an actor and his parents never forgave him. --------------------- See also: black, sheep Cambridge Idioms Dictionary, 2nd ed. Copyright © Cambridge University Press 2006. Reproduced with permission.

To be the black sheep means that someone that does not fit in with the rest of the family. Although it can have a negative connotation, it doesn't always mean that they are a bad person. If you come from a family of musicians, but you are a scientist, you can be called the black sheep.

I also see the word "a black sheep" in the song "Seasons in the Sun". "Goodbye papa, please pray for me I was the black sheep of the family". So I personally guess we can use this word for something that is different but in negative meanings.