I think we are again making a broad brush observation here. General Pat Cleburne was one of the first to call for the induction of Negro into the Confederate army. His proposal was endorsed by several other including Gen R.E. Lee. One of Cleburne's stated reasons why he proposed this was that the Union Army was using them in place of white soldiers.
Again this was a cause and effect reaction. The Union army wanted to reduce white casualities so as to negate Northern anti war sentiments. The Blacks in Contriband camps were willing to do anything to make a living.
Slaves wanting freedom may have been an element, but it would not have come about without these other factors being in play. Cleburne's reasoning was that if the Confederacy didn't do something to counter the use of negros in the Union army and refill the ranks of the Confederate Army, they were going to lose the war and the slaves would be freed anyway. Quite practical reasoning if you ask me, and does not seem to have anything to do with realizing "old Myths were wrong".