black sheep is usually used in the phrase "the black sheep of the family." Most sheep are white. Black sheep used to be considered less valuable because their wool wasn't the normal color. The black sheep is a person who doesn't fit the normal pattern, a non-conformist or an outcast.
"Black horse" I've never heard. Maybe you mean "dark horse". A dark horse originally was a horse not expected to win a race. Now it usually means a political candidate who seems to be the "underdog" (least likely to win), but has a chance to "come from behind" and win the election.