After reviewing you post I realize that I did not answer your question.
The time would be 1861, you do know that the CSA had the first Negro unit, the 1st La. right?
Also I should mention that most Northern states also had Black Codes.
I have never found a specific law that prohibits slaves or free Negroes from serving in armed ranks, but on the other hand I have never found a specific law that allows for such service either. If you look over some of the pensions and articles I have posted you will clearly that Negroes have served the Confederacy since day one.
Did I answer your question?