Since Daniel Blackburn, Forrest's Cavalry took the Oath of Allegiance to the U. S. May 1863, and you indicate that he enlisted December 10, 1863, I suspect he is the same Daniel Blackburn who enlisted in Company E, 12th Tennessee Cavalry, United States Army.

I would suggest you procure these records which could verify this. These may be procured at: http://history-sites.com/research/index.html

Private Daniel B. Blackburn, Company C, Blacks 5th Battalion Partisan Rangers, Forrest's Cavalry, CSA, "was taken under arms and after a pretty brisk skirmish" at Trenton, Tennessee, April 19, 1863, released on taking the Oath May 19, 1863 at the Military Prison, Alton, Illinois