I'm not convinced that all of the records from the Confederate Army and/or Government were saved. We know of the many courthouse fires that destroyed 'evidence'. We know there was a lot of confusion after Lee met Grant. I would assume there's a good chance that a lot of paperwork was tossed on the fire and we have no way of knowing just what it all contained.
Perhaps during some of the fighting the drovers or other Blacks that were 'there' took up arms and it was for self preservation during those incidents. It doesn't make sense to me that, other than a few exceptions, there were slaves doing any fighting in a formal, trained format. Unless someone pulls a rabbit out of the hat, we're not going to see any proof that could stand up to real scrutiny on the matter. It could have happened, but we have no primary source to show it did. I'm still thinking the pre-war activity by the abolitionist in trying to get the slaves to harm their owner is a sound reason for them to reject that on a large scale. Considering the ordinary infantrymans day on the battle field, if there were armed slaves, they probably would have had to watch their front and their back. Since a large number of them did not own slaves, that may have been asking a bit much for the common soldier.