at such a small size it is heard to tell if you actually have a rhombeus. most serras look a like when they are that small. but some closer shots might help with an ID on the little guy.
It looks like a black rhomb. to me! Thats what myne looked like when they were that small. Why are you worried that it's not a black? I guess only time will tell that tale, but i wouldn't worry much it looks good to me.