When the hobbits get caught in the Barrow Downs by the Barrow Wights, Merry seems to have a vision where he gets killed by a wicked man of Carn Dum. Is it possible these men were black Numenoreans? If not, who were they?
According to the Tolkien Gateway, the Men of Angmar (and therefore of Carn Dûm - since Carn Dûm was the capital of Angmar I'm going to use the two interchangably) were not Black Númenoreans, but rather descended from the Easterlings of the First Age, presumably the people of Ulfang the Black (who betrayed Maedhros in the Nírnaeth Arnoediad). I'm not aware of any actual writing by Tolkien himself to support this claim, however.
Update: 24th February 2015
A footnote to the Grey Annals (History of Middle-earth 11) notes:
It was after thought that the people of Ulfang were already secretly in the service of Morgoth ere they came to Beleriand. Not so the people of Bor, who were worthy folk and tillers of the earth. Of them, it is said, came the most ancient of the Men that dwelt in the north of Eriador in the Second Age and [? read in] after-days.
This is a possible source for the claim that they were descended from the Easterlings of the First Age, although we should note that this footnote explicitly states the people of Bor were the ancestors. It is however the only such authorial reference I can locate.
The Encyclopedia of Arda entry for Carn Dûm makes the following interesting observation:
A final possibility is that Carn Dûm comes from a Mannish language. Tolkien used real languages to represent the Mannish languages in his books, and especially Old English and Old Norse. Carn Dûm doesn't seem to belong to either of these, but there is another candidate: carn dúm are words from Gaelic that can be translated 'mountain fortress'.
Going by Tolkien's usual linguistic model, this - if true - would suggest that he wanted to suggest to his readers that the Men of Angmar were intended to be reminiscent of the older Celtic inhabitants of the British Isles.
However, it need not be the case that the Men of Angmar or of Carn Dûm were of any one particular culture or ethnicity. Lord of the Rings Appendix A notes the following (with my emphasis):
For at that time the realm of Angmar arose in the North beyond the Ettenmoors. Its lands lay on both sides of the Mountains, and there were gathered many evil men, and Orcs, and other fell creatures.
This suggests that the Men of Angmar came from more than one source, and were brought together from those sources to Angmar, and so there is likely to be a mixture of Easterlings, renegade (but not Black - see below) Númenoreans, Hill-men from Rhudaur, Dunlendings, and others. This is the most likely explanation as Eriador had historically been a crossroads between the East and West, with many different kinds of Men going back and forth and settling in different parts of it (Appendix F, for example, relates the Men of Bree to the Dunlendings, even though considerable distance separated them by the end of the Third Age).
The "Men of Carn Dûm" that Merry dreams of are therefore most likely to be quite a mixed bunch rather than a single group. The one thing we can almost definitely say for certain is that there is very probably not any Black Númenoreans among them.
Black Númenoreans are dealt with in the following question: How far south did Black Numenoreans reside? and it's almost certain that there were no major Black Númenorean settlements north of Umbar.