If I had to guess (I don't think you'll find an answer in LoTR), I'd agree with you that they were Black Numenoreans, though I do wonder how many still survived at this time. Perhaps they also included elite cavalry of Harad and the Easterlings who 'worked' for Sauron.
Of course Minas Morgul also had its own cavalry force, seen by Frodo as they marched out through Morgul Vale. I'd thought of these as the Witchking's guard cavalry, probably 'crueller than any orc' and maybe tasked with driving the hordes of orcs into battle.
After all, if Saruman had more sense than to trust his gates to the orcs, surely Sauron did too?
__________________
Rumil of Coedhirion
|