If Black Numenoreans are ever to be added I think there should be 2 variants. The first one being the Black Numenoreans in Mordor. They should be dressed in all black, have pale skin, and looking kinda like the Mouth of Sauron.
The second type being the ones that lived in Umbar. They should look somewhat like a mix between a man from Gondor and a Southron. But that's just my take.