Sure, in real life, an African-American wouldn't have been allowed in combat in WWII
Besides, there is really nothing intinsic about the modern Nick Fury that states he must be played by a white man. If they were doing the WW2 "Sgt. Fury" version of character, then race might be an issue, but as long as the character is well-written and portrayed, I wouldn't care what race of actor portrayed him in the modern context.
, but then, in real life, a soldier from WWII wouldn't have had his aging artificially slowed so he'd still be young and vigorous in the 2010s. So one shouldn't demand absolute realism anyway.
I don't know if you misstated that or not, but that's incorrect. African American men were indeed allowed into combat during World War II. The 92nd Infantry Division was composed mainly of African American soldiers (with white senior officers, of course) and saw heavy action in Italy. The 93rd ID was another African American unit, and they fought in the Pacific. And of course there were also the Tuskegee Airmen.
The Samuel L. Jackson version of Nick Fury still could have served in World War II, but unless he was part of another special unit like the Howling Commandos, he probably would have been with the 92nd or a fictional African American unit. Then again, in Captain America: The First Avenger
, they showed many African American soldiers integrated with the white soldiers, so like you said, there is a precedent there.