In "the return of the king" the Black numenoreans are the Black Corsairs of Umbar. Those of the "kings men" that survived the downfall of Numenor became corsair pirates, they settled in Umbar and made war on gondor with a rich history. Check these link for more info:

http://tolkiengateway.net/wiki/Corsairs

http://tolkiengateway.net/wiki/The_Mouth_of_Sauron

http://tolkiengateway.net/wiki/Black...men%C3%B3reans