I would hardly expect Frederick Douglass to be an expert on the confederate military and he did have an agenda besides. What better way to shame the North into recruiting black combat troops than to claim that the confederacy, of all places, already had them? And while you have occasionally posted alleged accounts from Union officers about black combat troops how about some accounts from southern leaders on black combat troops? How about Jackson talking about those 2000 black troopers who were supposed to be with his corps or Lee commenting on the battlefield prowess of the tens of thousands of black troops stande waite seems to believe were in the Army of Northern Virginia. Instead you have Lee relunctantly saying that although he would prefer a white army, the need for troops is so great that he is prepared to accept black ones.
There were even blacks who rode with Confederate Gen. Nathan Bedford Forrest when he gave Union generals fits in Mississippi and Tennessee. "Better Confederates did not live," Forrest said of the blacks that rode with him
I assume a quote from the man who is mistakenly credited with starting the Klan will be good enough? BTW, you may want to read that second link. A descendant of the Black Confederates made a documentary about them