Exactly, It would seem that the Confederate Army was far more "intergrated and multi-cultural" than the popular opinion would seem to want to reconize. The fact is that the Confederate army could not have stand in the field fighting for more than 6 months without the assistance of loyal black southerners even if they were slaves. The southern black were the southern armys logisitical support and commissary department that is without argument. The debate is whether those blacks were soldiers or not? The south reconized them after the war as being soldiers with pensions. Ad we still reconized men in those functions today as being soldiers during WWi nad WW II. But the southern black also took up arms where they were allowed to and fought side by side with the white confederate soldiers.
Frederick Douglas comments were correct at the time, but political positioning now requires that we deny the existance of such things, because it does not fit with the political agenda and rational for fight a war against the southern people.