1. Another common saying is that every family has one of these...
2. Think animals...
3. Think about farm animals...and unusual colors for them.

1. The odd one out

Since black sheep are generally rare compared to white sheep, to say that somebody is the black sheep of the family is to say that he or she is 'the odd one out': they are a bit different from the rest. Context: For example, say you came from a family who lived in a rural area, and were farmers for decades. If your brother wanted to leave the farm and become a businessman, you might say that he was the 'black sheep of the family'; since his viewpoints and lifestyle are different from the rest of the family's.