[Kalimac]

I don't think the Mouth of Sauron was necessarily that old, even though he had forgotten his name; I think all it says in ROTK is something like "of his name there is no record, for he himself had forgotten it." This didn't suggest age so much to me as brainwashing; he had let his whole soul be co-opted by Sauron in exchange for favors, so he truly was by this point a sort of extension of Sauron and not a real individual anymore. Hence the name "Mouth of Sauron." If his personality had been "erased" or he was brainwashed by Sauron (albeit voluntarily) it doesn't seem like forgetting his name, or even making himself forget it, would be too unlikely an occurrence.

As for why he's allowed to speak Sauron's name (after Aragorn says that Sauron "does not permit it to be spelt or spoken") I'd guess that originally this was just an inconsistency on Tolkien's part. He went through a lot of drafts on LOTR and since it's impossible to remember every throwaway line you write he probably just forgot what he had written earlier. But if we're trying to make it consistent, probably you could say that the Mouth of Sauron is unique among the servants of Sauron in not really being his own creature. Even the lowliest Orcs have their own names, and their own personalities, such as they are. But the Mouth of Sauron has ceded EVERYTHING to him; he has no name, no soul, no personality or will except that of his master, and essentially all he does is have Sauron's wishes and words channeled through him. So in a sense you could say that when the Mouth of Sauron speaks it's really Sauron speaking through him, and of course Sauron could refer to himself by name, even if he doesn't allow others to. (If you really want to nitpick, you could say the movie made a mistake on this point; in the scene where Saruman is shown speaking to the Palantir and says "What is your desire, Sauron Lord of the Earth?" Unless of course Sauron is permitting Saruman that liberty to gull him into thinking that he's looked on as an ally, and not a servant).

As for the "Black Numenorean" aspect of it. I never thought this meant he was particularly dark-skinned, more that it was one of those delineations like "Black Russian" (as opposed to "White Russian") where it has as much to do with where in Numenor he lived or what tribe/political affiliation he was brought up with as with the way he looked.