Michael C. Williams-
"I have yet to see any evidence of free blacks fighting for the South."

There were individuals who enlisted and fought in the volunteer companies and regiments.
Jack Thomas (TN), James Humbles (VA), Levin Graham (TN), John Buckner (SC) to name a few.

A battalion of black soldiers organized in March 1865 fought during the Appomattox Campaign
(see account of R. M. Doswell). Another unit organized from hospital workers was out on the
front lines on at least two occasions.

There's also a possibility that some of the free black militia units of LA fought with Confederate forces.