The argument as to slaves or FMoC being enrolled unofficially has been made elsewhere, so your point is understood. My counterpoint is that white Southerners, already sensitive to the idea of putting arms in the hands of black men, were even more troubled about it after Harpers Ferry. Even one black slave or FMoC, armed and uniformed, would have been highly unusual if not unique in any white military company. So it wasn't like you could just slip a black man into the ranks and no one would notice or particularly care.

Sensitivity towards race should be apparent in General Maury's request that a single company of Creoles be allowed into Confederate service. I have also already mentioned Alabama Confederates who were discharged on account of race. These were men who had served in their companies for some time, but were sent home due to questions about their ancestry. Remember that there were serious reservations in the North about arming black men. Race was a big deal on both sides of the Mason-Dixon line.

Try to find examples of slaves or FMoC enrolling in Northern regiments (not U.S.C.T.). At the time of the Civil War, it just wasn't done.

There might have been occasions in which trusted body servants might have picked up arms and participated in an engagement. Just my opinion - can't prove it one way or another. That may be what you had in mind as well.

The scene described in Frederick MD during Lee's first campaign north of the Potomac sounds reasonable. Except for the part about black men being armed to the teeth, of course. It could be that from a distance a Northern observer might mistake dark-skinned Confederate veterans with black men. It could be easier to make that mistake with body servants, teamsters and cooks moving along with the column of soldiers.

Whether we agree about this or not, it's nice to have a civil discussion.