Is the black clown an Ocellaris? Does it have the same color eyes as the percula? You shouldnt mix ocellaris with percula, percs are mean fish and will continue to bully any ocellaris until theyre dead.
If they are both percula, then they might be both female.
regardless the fish need to be separated. When trying to pair clowns ensure theyre the same species, and get a male from a mixed tank of clowns at an LFS. I always have separated females, confirmed pairs and a mixed tank of unclaimed males and one female at work. The largest of the group is the female, she will be more aggressive and hold her dorsal fin higher, the males will be smaller and docile.