沪友详解A：
a black sheep 是害群之马的意思（不是黑羊哦）
第一次听到是在westlife的seasons in the sun里面有一句：
I was the black sheep of the family...

沪友详解B：
black sheep=a person who has done something bad which brings embarrassment or disrespect to their family.

例句
1、Black sheep dwell in every fold.
每个团体都有害群之马。
2、There are black sheep in every flock.
害群之马到处有。
3、Perhaps every profession has got its black sheep.
也许每个行业都有害群之马。
4、There is a black sheep in every flock.
[谚]人多必有败类。
5、He is regarded as the black sheep of the family.
他被认为是个败家子。