like others said, it's not just black birts, it's across the board with brit/euro/aussie talent. that said, it stings more due to there being fewer roles for black actors, especially when you get into leading or prestige roles, and that decreases even further for black actresses.
a lot of it has to do with our, currently dying, system of looking for stars. for too many years, getting on in hollywood was more about a look, a break, and who they thought was bankable, talent only got you so far. it's why we always had a clearly transcendent level of talent (denzel, pacino, streep, hanks, leo) etc. and then "stars" some of whom were in that A class and others who were stars simply because of the success of their films and "other" intangibles. overseas, they tend to have a lot of great talent packed into their "unknown" tier...we don't, most of that tier for us is people who either picked up acting late/as a pivot (the rock, channing tatum) and aren't classically trained or they started on TV which often limits your breadth due to the nature of just playing one character.