There's a million plausible reasons for each of the black contestants to have been so unpopular with the public and we'll never know one way or the other whether their skin colour had anything to do with it.
But the fact that all 3 black contestants ended up in the dance off in the first 2 weeks is quite striking and worthy of comment IMO.
Out of 12 non-black celebs, just 1 was in the dance off in the last 2 weeks. Of the 3 black celebs, every single one of them was.
As I said, there's no proof of race playing any part in the results but it's fair enough for people to notice and remark on it.