You avoided to address the Southern treatment of black Union Soldiers of War. Southerners stated they would treat captured black soldiers as they would treat run away slaves, execute them or send them back to the cotton fields. Sorta like the Germans sending American Jewish prisoners of war to the Concentration Camps instead of the Stalag.
And how exactly did northerners treat black Confederates?
There is no record I know of a single black confederate soldier being captured by Union Forces. There may have been, but I know of none. You can probably bet that they would not have been treated like run-away slave were treated in the South. Run-away slaves in the Union could enlist in the Union Army.