A reminder that our frame story is set in Harad, well to the south of Gondor. The decision is to have all the characters of Harad be black (of African descent). This includes any families with Numenorean blood - the Numenoreans were active in the region over 3,000 years previously, so at this point, they no longer look any different from their neighbors.
So, all 4 of our frame characters will need to be cast as black actors - the Mouth of Sauron, the Queen, and the two young adult princes. It was important to Corey Olsen that the Mouth of Sauron look to be of the same people as the residents of the seaside town in Harad that will serve as our 'Hobbiton-of-the-South,' while Gandalf is the clear outsider (based on appearance).
We have not yet made any decisions about the Nazgûl; their origin is at a different point in the history, so we may make different decisions then.