Renee Ingram in her book In View of the Great Want of Labor A Legislative History of the African American Conscription in the Confederacy is a wonderful source.

It includes chronological excerpts of discussions, from the Journal of the Congress of Confederate States of America, regarding the employment slaves and free blacks during the Civil War.

As her focus is Virgina..included is the Register of Free Negroes (Males) Enrolled and Detailed (in VA) May 1864 to Jan 1865

Example:
John Copeland age 37; eyes: black, Hair: black; Complex: black; 5 feet 7 in.; born: VA; town or county: Botetourt; Occupation: boatman; when enlisted: 27 Jul; Where: Fincastle; by whom: Lt. Nowlin; where assigned: James River & Kanawha Canal Company

These were not voluntary.

Selma