You're very thoughtful, and I'm pleased to have been able to help you.
If you review Dr. Levine's statements, he's not denying there were "ANY black Confederates" - just no C.S.A. black combat commands. BTW if you start looking in March 1865, "there were organized, officially sanctioned units of black soldiers in the Confederate Army." Levine mentions them, and in March-April 1865, numerous requests were written by Confederate officers to organize and lead black units.
Even "given the well known official position of the Confederate government," rank and file Confederate soldiers should have mentioned black comrades during the war. It's not like Confederate diaries and memoirs are rare. People don't seem interested in Confederate accounts - they prefer to hear from Yankees about "armed picked Negroes" and what-not.
Cleburne, Ewell, Oates and others had no firm basis for their belief that Black Confederates could perform effective service. They simply saw the manpower pool of the South running out and knew that new approaches to the problem would need to be advanced. If Cleburne actually saw "some of Forrest's crack black troops," the natural question would have to be, "How come he can do that, and I can't?"