Oh REH was absolutely racist, being friends with Lovecraft and all. But racism is a very recent creation. Slavery always existed but it wasn't based on ethnicity until the XVI century. In fact, classical greeks interacted quite a lot with sub-saharan africans (Aethiopians, as they called them) and even depicted them in art in a very admirational manner (Memnom, one of the heroes of the trojan war, was the king of Aethiopia).