Historian Bruce Levine has a really good op-ed looking at the myth of black confederate legions. Here's Levine discussing the Confederacy's death-bed conversion:
After months of heated debate, a severely watered-down version of this proposal became Confederate law in March of 1865. Gen. Richard S. Ewell assumed responsibility for implementing it, and Confederate officials and journalists confidently predicted the enlistment of thousands. But the actual results proved bitterly disappointing. A dwarf company or two of black hospital workers was attached to a unit of a local Richmond home guard just a few weeks before the war's end.The regular Confederate army apparently managed to recruit another 40 to 60 men -- men whom it drilled, fed, and housed at military prison facilities under the watchful eyes of military police and wardens -- reflecting how little confidence the government and army had in the loyalty of their last-minute recruits. This strikingly unsuccessful last-ditch effort, furthermore, constituted the sole exception to the Confederacy's steadfast refusal to employ African American soldiers. As Gen. Ewell's longtime aide-de-camp, Maj. George Campbell Brown, later affirmed, the handful of black soldiers mustered in Richmond in 1865 were "the first and only black troops used on our side."
In his book, Confederate Emancipation, Levine goes into some detail about the fate of some of these troops:
Not 4,700 but have a dozen free black males enlisted...One, a boatman named John Scott, had appeared at the recruitment office bristling with hostility toward the Yankees. After enlisting, he seemed for all the world to be an enthusiastic soldier and an apt student of military drill. But then, early one Sunday morning, Scott gathered up some twenty-five full uniforms and (the Examiner reported drily) "putting in brilliant practice one of the military movements, the 'double quick' decamped." Pursued the erstwhile soldier, by "executing another dexterous movement, the 'right wheel' eluded his pursuers and escaped.'"...the Richmond Whig advised Major Turner against putting any more faith in the "patriotic" pretensions of "negro boatmen, all of whom, are sharp fellows and can't be trusted."
One other point I noticed while going back over Levine's book: 90 percent of all Confederate congressmen were slave-owners. 40 percent were full-fledged planters--meaning they owned twenty or more slaves. It's worth remembering that slaves were property, and extremely valuable. A planter sending his slaves to war did not simply have to consider the societal repercussions--and there were many--but the financial repercussions also. It would be like handing your Landrover over to the government to be turned into tank, all the while understanding that your SUV would not be coming back. Indeed, you'd effectively be sacrificing your Landrover for the cause of banning SUVs.