Black sheep is commonly used to describe a member of a family who is a disgrace or who does bad things or is a failure.
由此可见,black sheep并非完全用于贬义,也可以用于形容不幸。例如说,一家人都很成功,但某一个孩子只是做普通的工作,经济上仅能果腹,这时候可以说 He is a black sheep in the family. 这句话,只是形容家庭某一成员不幸,而非骂人的意思。