There's a difference between being black and being Japanese. Being Japanese means having Japanese traditions, speaking Japanese as mother language etc. - belonging to the Japanese culture and often having Japanese parents. (I know this in not a good definition at all, but I hope you got me)
Black means just having black skin. This information doesn't tell you anything about someone's home country, native language and so on.
I know two German/German-Finnish Naomis (or ok - one is Neomi and another one Noomi). I really like
Naomi phonetically, but prefer
Noemi or Noémie. It is a little cutesy name that looks special, I'm not sure, in a good or in a bad way :). Completely useable.