I suspect that this has nothing to do with the Africans being African. It's just a case of 'our citizens first, the rest of you (wherever you're from) will just have to take your turn.'
I saw this on one of the news programmes. The young Africans were students from the Universities in Ukraine. I hope that they managed to eventually get on one of the trains.
It did actually say in the article that only women and children were being allowed to board. As these students fell into neither of those categories, I can't see that the fact that they were foreign was at all relevant.