Of course, we’ve seen other black guys like Mace and Lando, so it’s not like we haven’t had black guys in there before. (Indeed, Vader was once voiced by a black guy.)
As to the idea that the introduction of the black stormtrooper is a form of political correctness, there might be some truth in that. Indeed, the way the Empire has been laid out, it would seem very odd indeed.
The Empire is pro-human. It believes aliens like Wookies are lesser beings. Thus it used this xenophobia to justify its enslavement of non-humans. While the Alliance has non-humans like Akbar in it, the Empire is all human (at least that we’ve seen. Not sure if they had any non-humans in the recent “Rebels” series.)
Also, it’s pretty much sexist too. While there might be some, like Admiral Dahla (though she’s more non-canon than canon.), it’s pretty much all men in the Empire’s ranks. The Alliance, on the other hand, has those like Leia in the leadership.
The Empire also does appear to be racist. At least as far as we’ve seen, till this new movie coming out, all of the Empire’s leadership has also been white. The Alliance, however, has had those like Lando.
Indeed, uniformity seems to be the RULE of the Empire. They have their army all look alike (clones). They seem to be composed of white human males in their leadership. And if anyone disagrees with how they run the place, they blow up their planet.
In fact, the term stormtrooper referred to a part of Hitler’s fighting force. Hitler believed in an Aryian race and was horribly racist and anti-diversity, much like the Galactic Empire.
While the Empire appears to be the picture of uniformity and conformity, the Rebel Alliance seems to be the picture of diversity and free thought. One would think that the Empire in the new movie would be the same way and that the Republic, which I think is back and was created by the Alliance, would be like the Alliance. The Alliance at least in the cannon, seems to be anti-genocide, etc while the Empire, which it demonstrated quite clearly by blowing up Alderan, is the complete opposite.
Thus, the black stormtrooper, while a cool concept, seems strange. Then again, maybe the white male human policy was specific to the Palpatine Administration and whoever is in charge of the Empire now is a bit more diverse than Sidious. Whether that is indeed the case, or whether this is just a PC move remains to be seen.