I think the best compromise for this unit is to include extra heads in the American Civil War sets, that have African features. Then the purchaser can use those heads to create new units of Black soldiers.
I never paint my figures so I appreciate when a sculptor makes the figures details look like they should, Whites white, Blacks black, Asians Asian, Mexicans Mexican, etc.
I especially love the Strelets ACW Command Sets because Custer looks like Custer, Grant looks like Grant, Lee looks like Lee, etc.
I'm always looking for accurate facial details, and also helmet/hat details, and they never go unnoticed by me. I've always loved Strelets sets ever since the first Charge of the Light Brigade Cavalry sets came out. Like the song (kinda/sorta) goes, "Strelets had me at the first glance of helmets and facial details." Any details below there didn't matter.
So, if a set were dedicated as the 54th Massachusetts Volunteer Infantry then they should look like African Americans, or if not, then as Mike says, I'd love to see heads as accessories.
And, the Buffalo Soldiers would be a great future set also. Super suggestion!
when doing a conflict, one should start with basic sets first, that gives impression of customers' demand to this conflict. That's a lesson we learned, when we started Crimean war series. Should the series prove to be a success, we can expand in nearby areas.