In the English language, black sheep is an idiom used to describe an odd or disreputable member of a group, especially within a family. The term has typically been given negative implications, implying waywardness. It derived from the atypical and unwanted presence of other black individuals in herds of white sheep.I looked this up this morning because of a report in the Mail Online of a English flock of 37 white sheep that have just given birth to 60 lambs, all of whom are black.
The idiom is also found in other languages, e.g., French, Serbian, Bulgarian, Hebrew, Portuguese, Bosnian, Greek, Turkish, Dutch, Swedish, Danish Spanish, Czech, Slovak, Romanian and Polish. There is an expression "white crow" ("белая ворона", "belaya vorona") in Russian, in Persian there is similar expression "white crow", "kalag-e sefid" (کلاغ سفید), which roughly means black sheep.
The term originated from the occasional black sheep which are born into a herd of white sheep due to a genetic process of recessive traits. Black wool was considered commercially undesirable because it could not be dyed. In 18th and 19th century England, the black color of the sheep was seen as the mark of the devil.