If someone’s a Black Sheep, it means they are (Different from the rest of the people in a group or different from the rest of their family)
Think about is. Most sheep are white. Imagine two sheep had a lot of baby sheep. Now imagine all of the baby sheep are white, except you have one black baby sheep. That baby is very different from the others.
When we say that someone is a black sheep, we are saying that this person is very different from the rest of the family or group. The black sheep makes their own decisions and lives a totally different lifestyle than the rest of the family.
So a black sheep is different, special (could be a “good special” or a “bad special”), and unique.
Ideas from: http://www.speak-fluent-english.com/