In The Avengers: Earth's Mightiest Heroes, it was Nick Fury's father Jack Fury who was a member of the Howling Commandos. No explanation how a black man could've been in combat in WWII, but then, it was an alternate history where WWII was fought against HYDRA rather than the Nazis.
Come to think of it, didn't Gabe Jones appear in Captain America: The First Avenger? So there's precedent in the MCU.
There were black men (entire units even) in combat during WWII, especially in the latter half of the war. They just weren't in white units.