Consider the source. They are usually up front about their organization producing the information at hand. IMHO this is an example of the SCV delving into contemporary politics. If armed Black Confederate volunteers really existed, the SCV feels it can "prove" that the Civil War had nothing to do with slavery - 'nuff said; case closed.
Now, ask a follow-up question like this - let's just say that slavery didn't exist anywhere in the United States In 1860. If Lincoln is elected (and the election campaign would have been very different without slavery), do the Southern states still decide to leave the Union. If the answer is yes, the question is why? We fight a bloody, destructive war for four years and slavery doesn't exist. Please explain what grievance would prompt the Southern states to secede?
Wyeth's That Devil Forrest is such a classic of historical writing. I've owned a copy since my seventeenth birthday, and have read every page many times. If there is a reference to armed Black Confederates in any of Wyeth's pages, it has escaped my attention.
You'll really enjoy this book!