Found in the Gerdes site: Missouri Cav-Freeman's Regt.
"Blackburn, Robert O - CO D - Pvt. - Captured May 20, 1864, Jackson County, Ark, died
September 4, 1864 in Little Rock, Ark. Military Prison."
At the opening of the Arkansas site up toward the top of the page, check that area and
then enter. That is where I found the information in the Gerdes site.
I believe that Blackburn will be found in a book called Rebel's Hell which was written
in the early 1990's. It was taken from the National Archives just like the information for
the Gerdes site. I found my own ancestor there.
To my knowledge, there are no copies of Rebel's Hell still available for sell, but it would
be possible to get one through an inter-library loan. The advantage to doing that is that
sometimes you can figure out who the people were with him and then even track down
who captured him.
The site of the prison was where the Arkansas state capitol sits now.
More than likely Blackburn was buried first at Mt. Holly and then moved over to the outside
of the National Cemetery. More than likely he is one of the many "Unknown CSA"
I know this because that is exactly what happened to my ancestor,
Lt. Fielding Price, as far as I can piece it together.
Hopes this helps.
Linda