"Black Muslim" is a lot different from "Muslim."
"Black Muslim" groups like Nation of Islam are not even considered to be Muslim by your garden-variety Muslims. They have fringe beliefs, such as considering their founder Elijah Mohammed a prophet.
So, that's why it matters. Now, there are bona fide Muslims who happen to be black, but the use of "Black Muslim" leads me to believe this is a member of one of these American fringe groups, who, I think, still make up the bulk of American Muslims.