We Nine Kings of....where?

[Rimbaud]

Certainly a very patchy area. The only explicitly named, as said above, are the Witch-king of Angmar, and his second in command Khamûl.

At least three of them were of Black Númenórean race.

Khamûl was a lord of Easterlings, and was the only Nazgûl known by his name, although there has been some speculation that Gothmog, Lieutenant of Barad-dûr, may have been a Nazgûl. That said, Gothmog generally isn't an especially clear part of the canon, what with Gondolin-Goth as well...