If I may attempt to answer your question of why to prove the existence of Black Confederates.
First of all our opponents speak of the idea as if it is something of a rarity, scarce as hen's teeth if you will. This is nothing but a myth, to date, with the help of people like Glenn, SHAPE has documented a little over 10,000 Blacks who have some sort of service with the Confederacy. These include are all sorts of records, newspaper accounts and letters.
A very good reason to prove this point is just look around at all the battles we lose because we are "racist" or that our ancestors were fighting to preserve the institution. To prove the existence of Black Confederates helps to put these lies to bed. Also this is part of the truth about the WBTS that is hardly ever studied, it is the part of a myth those Yankees will fight tooth and nail to try and prove no such thing as a Black Confederate existed. In my opinion this issue is just as important as marking a Confederates grave or making any reasonable study of any aspect of the WBTS.
Hope this helps.