This article isn’t particularly well written, but it did answer a question I had upon first reading it: Black troops got no bounty rewards, so I suppose there were no black substitutes.
Before the war would end, 180,000 blacks would take up guns and battle for their freedom.
But, while recruitment was easy at first, it grew harder. Even though they were accepted as soldiers, blacks were not given a fair shake, and potential volunteers knew it. Blacks could not serve as commissioned officers, for instance: For the next several score of years, they would always be ordered around by whites. They received less pay than their Caucasian counterparts and also were denied the sign-up bounty that whites received.
Worst of all, the Confederacy threatened to execute or enslave every black man captured and to execute every white officer over them.
Even Douglass quit trying to raise troops until he was convinced Lincoln was doing something to solve these issues.
via Bill Hand: The first black troops in the Civil War weren’t always easy to recruit – Local Columns – Sun Journal.