Here's the part of the Steiner report that mentions black Confederates.
The only item worthy of notice or debate would be-
"Most of the negroes had arms, rifles, muskets, sabres, bowie-knives, dirks, etc."
Nothing unusual about the rest. 3,000 black servants, teamsters, cooks intermingled within a Confederate army would be rather typical.
"Wednesday September 10 .—At four o’clock this morning the rebel
army began to move from our town, Jackson’s force taking
the advance. The movement continued until eight o’clock p.m.,
occupying sixteen hours. The most liberal calculations could not
give them more than 64,000 men. Over 3,000 negroes must be
included in this number. These were clad in all kinds of uniforms,
not only in cast-off or captured United States uniforms, but
in coats with Southern buttons, State buttons, etc. These were
shabby, but not shabbier or seedier than those worn by white
men in the rebel ranks. Most of the negroes had arms, rifles,
muskets, sabres, bowie-knives, dirks, etc. They were supplied, in
many instances, with knapsacks, haversacks, canteens, etc., and
were manifestly an integral portion of the Southern Confederacy
Army. They were seen riding on horses and mules, driving wagons,
riding on caissons, in ambulances, with the staff of Generals,
and promiscuously mixed up with all the rebel horde. The fact was
patent, and rather, interesting when considered in connection
with the horror rebels express at the suggestion of black soldiers
being employed for the National defence."
In most or all Southern states it was against the law for a slave to carry or use a weapon...except if it was in defense of the master or his property. I believe this exception was actually written into the law in some states.
Steiner's description of these servants ("bodyguards" of their masters) carrying weapons may be correct.