"using black troops as cannon fodder"

General Grant wasn't the only one to use that tactic.
Asa M. Piper, Co. C 62 Regt of Alabama Volunteers told in his "Recollections of an Old Soldier" of fighting at Fort Blakely on the 9th of April 1865.

"the U.S. troops charged our thin line with negroes first, and we slaughtered them fast, some 2000 being killed in a few mintes with ground torppedoes and by shot and shell from the breast works. Then we were charged by five lines of battle and captured...."

It seems as if it was a regular tatic of the Union Army.