Of free blacks, I know of these examples:
1) Jason Boone, freeborn black laborer with 41st Virginia Infantry, present at Gettysburg (Civil War News, Feb/Mar 2000)
2) Several free blacks were hired by the Quartermaster department of the 7th Virginia Cavalry as wagon drivers or teamsters (7th Virginia Cavalry, Virginia Regimental Histories Series)
3) James Humbles (colored), bugler. Born free in Virginia, enlisted at Lexington VA on 18 April 1861 as a bugler. Free negro, mustering officer thinks this man should be mustered in. On muster rolls at a mechanic for the water works in Lexington, 1861-1865. For some time during the war he carried his musket and saw active service. Died Lexington 28 Sep 1906, buried in Evergreen cemetery. (1st Virginia Infantry, Virginia Regimental Histories Series, roster)
4) 11 July, 1863, "I have to send this letter by a free Negro to Richmond. He says he cannot wait for me to write more." (Richard Henry Watkins Papers, 3rd Virginia Cavalry.