This tale might be confused with what had previously transpired.

When the war started, Forrest asked 45 of his slaves (which he considered as servants) to join him, offering them their freedom after the war, no matter how it turned out. They all joined him and although they had numerous opportunities to desert him, 44 stayed by his side until the end of the war. In fact, part of his special command escort later called "the green berets" (ironic isn't it), consisted of the most elite and best soldiers available, and among them were eight black men. (Surely you are aware that nearly 100,000 black men fought for the Confederacy?).
http://www.southernheritage411.com/truehistory.php?th=039

See also http://www.dixiescv.org/fact_did-blacks-serve.html