There were already quite a few black Southerners serving the army and navy in various capacities including as soldiers. One of the most notable was Louis Napoleon Nelson, a black Southerner who had fought in several major battles including Shiloh. Likewise there had been a regiment of free blacks from Louisiana who had volunteered and marched off to join the Confederates in the war. At one point I had a copy of a photograph of them in their gray uniforms. Perhaps I can find that again and post a link to it.
These things are often overlooked. It would do us well to learn more about them.