This report by Col. Allenbach made me think of a phrase I've often seen variations of in reference to black Confederates, found here:
https://www.battlefields.org/learn/articles/black-confederates
"To be specific, in the “Official Records of the War of the Rebellion,” a collection of military records from both sides which spans more than 50 volumes and more than 50,000 pages, there are a total of seven Union eyewitness reports of black Confederates. Three of these reports mention black men shooting at Union soldiers, one report mentions capturing a handful of armed black men along with some soldiers, and the other three reports mention seeing unarmed black laborers. There is no record of Union soldiers encountering an all-black line of battle or anything close to it. "
However, is that assertion not contradicted by the portion of the report quoted in the OP, when he encountered the enemy skirmish line? I don't know how many men were in this line, but there had to be enough to help the artillery stop two regiments of men. As for the composition, note that the colonel doesn't say "some" were negroes, he says "the skirmishers of the enemy were negroes." That sounds like something "close to" an all-black line of battle to me.
"In obedience to these orders, at about 11 o'clock I advanced with these two regiments forward through the wood, under a severe fire of shell, grape, and canister. I encountered their skirmishers when near the farther edge of the wood. Allow me to state that the skirmishers of the enemy were negroes. Slight skirmishing going on until retiring. "