Part of the "black Spanish" legend is in that in the New World, many of the early adventurers who "Were conquering in the Name Of God!" would have found their way into a noose in Spain. The early history of the Southwest is filled with examples of conquistadors getting in trouble with the local bishops and priests, and the priests were constantly writing to their overseas superiors about the various idiots running around enslaving and looting the local inhabitants.
The local Christians liked the priests (which is why many tribes overthrew the leaders of the revolt in the 1600's), didn't mind the representatives of the Spanish crown, but hated the looter (often from Spain). That started the black legend.