i stand by the statement that to mention the guys race is utterly and completely irrelevant
except that MTB is so overwhelmingly dominated by white (middle aged middle class IT) guys that a descriptor such as “black” puts him in a recognisable minority, hence the poster using it. Just as he wouldn’t have used any other descriptor unless it was notable and made him or her stand out from the crowd (eg. extreme height).
Mentioning colour is such a taboo it’s laughable; if there’s one guy in a group of 20 who is black, people will still describe him as “the guy towards the right with the blue jeans and red t shirt..no…the taller one…”.
You’re loading the (use of the) word with far more subtext and meaning.