A black youth has been denied a leadership post in a Mormon-operated Boy Scout troop because of a church policy that discriminates against blacks, the NAACP reported yesterday. . . Church spokesmen acknowledged last week that "one of the policies of the church in regard to scouting is that the deacon's quorum president also serves as the senior patrol leader. No other member of the scout troop, whether he is a member of the church or not, holds that position."