There were a few who did; blacks who served in the Civil War overwhelmingly chose to fight for the Union.
I know that now. After meeting that young man I read some books he recommended and recently I met a NC man who is researching the subject. I asked how his research had been received given the reaction we saw at the AHC and he said his response had been mixed.
There were masters who took their body servants with them into the Confederate army, and I think there were cases of those servants taking part in battles, but they wouldn't have been officially regarded as soldiers.
Another possibility would be persons who had some African ancestry but who were able to "pass" as white--maybe DNA would be the only way to determine that there was some African ancestry.