This site references the well-known quote by historian Ed Bearss. Even if Ed ever said these words, he's not saying anything questionable. Of course slaves and free men of color by tens of thousands supported the Confederate cause directly and indirectly. They didn't have to actually handle muskets and shoot Yankees to help. Even if this is an authentic quote, Ed doesn't say anything specific about slaves and FMC in actual military service.

http://cwmemory.com/2010/02/06/ed-bearss-on-black-confederates/