Most families have at least one embarrassing loafer who is referred to by the others, and sometimes by himself, as the “black sheep.”
A black sheep is considered worthless because, unlike the majority of sheep, its dark wool cannot be dyed.
Although it takes as much time and nurturing to raise a black sheep as it does any other, its wool has very little market value.
That makes raising it almost a waste of time to the shepherd.
In 18th and 19th century England, the black color of the sheep was seen as the mark of the devil.