who are the nazgul?

[Zigûr]

Originally Posted by Ivriniel

There is some dispute about the number and kind who were numenorean. Someone posted somewhere that Sauron nabbed some of them during his time on Numenor--but the dates don't reconcile for that.

The quote I gave from "Akallabêth" states definitively that there were three. They certainly did not receive their rings from Sauron while he was in Númenor because he didn't go there until much later, but as you've stated there were many Númenóreans in Middle-earth before that, hence my suggestion about "colonial lords."

Originally Posted by Ivriniel

I wonder if the Blue Wizards had any role in the process, who were Istari who fell into the 'evil of sorcery' in the East. We never find out if they were competitors, or non-events for Sauron--or allies.

For them to have been involved they would have to have come to Middle-earth in the Second Age, which was suggested but never settled upon by Professor Tolkien, and they would also have to have fallen to the shadow - another thing he never settled upon - and abandoned their mission extremely quickly. That being said, I can't see why they would need to be involved. Sauron already had plenty of influence in the East without the need for intermediaries.