Re: Quote of the Week from my gaming group...
In the 41st millennium you're either human or mutant. Most of the human race has homogenized down to a single "race", however, despite the game being created by and for English middle class white males in the 1980s, there are a few deviations from that white baseline that are noted.
Midas Betancore.
Magnus is given Egyptian trappings and described as having dark skin.
Tallarn Desert Raiders are Berber/Arabian.
Heqta Jajjo is described as dark skinned.
Space Marines drawn from the Achaemenid Empire were from the areas ruled by the Persian Empire on Earth.
There are examples, but as the writing and setting matures, you see more diversity (not unlike comics). In any case, with the exception of the Salamanders (as Susano mentioned), which are based on the story of John Henry, none of them would be "African" or "American" as those concepts are 39,000 years out of date. And, since the origins of the game are English, calling anyone African-American would get you a funny look, because the majority of sub-Saharan Africans of native origin (black people) living in non-African countries are not American.