Canonically? No one cares. The important thing is that you're human, it doesn't matter what colour you are or what chromosomes you have. The hellish nature of everyday existence in the Warhammer 40,000 universe has, ironically, actually been a real boon for racial and sexual politics, because everybody has more pressing things to worry about.
However, that's a bit of a
Thermian Argument, because honestly? You know and I know and everybody knows exactly why the artistic decision was made to depict that character as a black woman.
I don't actually have a problem with it, lorewise - the Astra Militarum ultimately just wants more meat for the grinder at the end of the day - but from a marketing perspective, I'm just noticing it more and more, and, frankly, it's starting to rankle and strike me as cynical pandering. I hasten to add that I'm not saying that I don't think there should EVER be a black protagonist or a female protagonist, it's just...
Well, it'd be nice to see a new Astra Militarum book come out from Black Library and it NOT have the most prominent person on the front cover be a woman in military fatigues.

Oh, but of course, she still has to be somewhat conventionally attractive, though, because they made Ursula Creed look like Miss Trunchbull, and nobody liked that.