Some USCT regiments were formed in areas where most of the recruits were ex slaves. I understand that these would have had white officers but where did the NCOs of the regiments come from? For a regiment to function properly a regiment would need sergeants who could read, write, and understand numbers. Although finding recruits with these skill might not be too difficult in areas where many of the recruits were free men of color, in areas where most or nearly all of the recruits were ex slaves, recruits who could read and write might be on the rare side. Some pre War Southern states did not allow teaching slaves to read and write. Some slave owners might teach their slaves to read and write despite the law. Still, I was wondering if USCT regiments recruited from former slaves could find enough educated former slaves to allow for the regiment to have sergeants who could read and write.
As you say, there were plenty of literate blacks in the North who could serve as NCOs in their regiments.
In the South, where the slaves had little or no education, they were often taught reading and writing while enlisted. It was very common for freed slaves to learn to read and write in the military, and continue their education after the war.
Presumably some of them learned quick enough to become NCOs, but I have no hard data on that. It is possible, but not certain from what I remember.