The Mouth of Sauron was also a Black Numenorean, a Numenorean who fell under the sway of Sauron. I doubt he was born in Numenor, probably he was just the descendant of the Numenoreans (perhaps of a garrison of soldiers and women in Umbar). Anyways, in the last years of Numenor the king captured Sauron and took him to Numenor. Sauron rose through the ranks, so to speak, and became the king's advisor. He corrupted the mind of the king, started a satanic religion, and was generally the real ruler of Numenor. The Black Numenoreans were those who followed him.
And I would read the books, if I were you, or our conversations might sound like nothing but elvish babble!
EDIT: Whoops, cross-posted with the last two.
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