LOL - the black soldier in the movie spoke so few words, I didn't even notice his accent!
That is correct - many of the men in the Northern colored units were literate. The 54th and 55th Mass regiments, one of which was featured in the movie Glory, were actually more literate than many white regiments (although that is not clear from the movie). It is fair to say that these men were nor representative of the entire colored troop population, but they made up a decent sized minority of that population, I believe.
In any event, it was not incorrect or unrealistic that a colored soldier might have talked as this black soldier in the movie did. He would have been one of what was commonly referred to as an "intelligent negro" at the time.
- Alan