[JeffF.]

"This black-clad cavalry are unlikely to be Southrons"

RotK states specifically that they were Haradrim.

"Southward beyond the road lay the main force of the Haradrim, and there their horsemen were gathered about the standard of their chieftain. And he looked out, and in the growing light he saw the banner of the king, and that it was far ahead of the battle with few men about it. Then he was filled with a red wrath and shouted aloud, and displaying his standard, black serpent upon scarlet"