Shelby Foote was not a trained historian and much of his "history" is based on second- or third-hand accounts that would not stand up to professional standards
he also, when asked, said that Black people didn't deserve reparations because they had already gotten them through crime
Tube Amps for Justice
Nearly 180,000 Black men served in the Union Army. Another 19,000 in the Navy. Black soldiers accounted for roughly 10% of Union military deaths. A large portion of each were formerly enslaved.
I imagine they gave a damn about slavery.