I do not think ‘black’ has anything to do with it. Carson is a man Trump has come to respect and appreciate. I also know that Dr. Carson (and others) have had a profound impact on P.E. Trump’s Christian growth and development.
If Trump doesn’t name any blacks to his cabinet there will be hell to pay. I think they mutually like and respect each other at this point, and Carson’s support was valuable to Trump through the campaign.
But he also checks an important box. That IMO is why Carson’s acceptance of the nomination is especially valuable.