There are not many African Americans in Bismarck, so there may not have been separate records.

There seems to be a number of microfilms of Burleigh county records available....have you tried ordering them in to your local FHC or library?

http://www.familysearch.org/Eng/Library/FHLC/frameset_fhlc.asp

Try a PLACE search for Burleigh, North Dakota