The phrase 'Black Sheep' is used for a person who fails join other in some common cause or one who betrays his associates.
Generally, the colour of a Sheep is white or brown. A sheep with black fleece is rarely visible. Hence the use of the phrase.
1. Ever since I have brought the irregularities in the notice of the principal, he has been branded as black sheep.
2. There is black sheep in every society.
3. Mr Mathur is a black sheep in his family.