Whenever we see someone in a pack of friends but he seems to not belong with them, we call that person a black sheep. Or when our favorite team has a player who doesn’t want to cooperate with the rest of the team. What about one of the siblings who doesn’t achieve any success and is a lazy bum unlike the rest of the family. Most families have at least one lazy person per family, that person is often called the black sheep. A black sheep is considered worthless, a normal sheep can produce wool that is useful. Nurturing a black sheep takes the same amount of time as a white one but the black wool has no value at all in the market. So raising a black sheep is a waste of time and often are considered useless.
Black wool was useless because it could not be dyed. So uselessness of these sheep created an analogy with useless humans.