Black sheep (of the family)

A black sheep is a person who causes shame or embarrassment within his or her family or peer group because of beliefs, behaviour etc that vary from accepted norms, dates in this sense from the late 18th century, supposedly because the wool from black sheep was worthless because it could not be dyed. On the other hand, the occasional black sheep in a flock were regarded by English sheep farmers as a good luck omen but the former negative connotation was the one that generally prevailed.