Since 1866 Southern education has been controlled by Northern interest. Why would the victorious North want to promote Confederate Blacks? Heck, until the movie Glory, the interest in America in the efforts and actions of Black Union soldiers was pretty much nill. The racial attitudes of Southerners in 1950s and 1960s were a product of Union Reconstruction and abandonment of Blacks by the Republicans.
During the Civil War Frederick Douglas knew the amounts of armed Black Confederates was enough to be concerned about and made note of it.