Was he aware that slaves and freedmen alike fought integrated on the Southern side? I reckon this shows that no human can ever be held too high, lest we fall into fallacy.
Dabney was for a time a staff officer of Stonewall Jackson, so he knew the composition of the army.
By the way, is Dabney referring to black soldiers on the North or the South when he states
"If indeed they can mix the blood of the heroes of Manassas with this vile stream from
the fens of Africa, then they will never again have occasion to tremble before the
righteous resistance of Virginia freemen; but will have a race supple and vile enough
to fill that position of political subjection, which they desire to fix on the South."?
He is not referring to Black soldiers. He is referring to laws which would lead to Blacks and Whites being able to marry and have children, which he thought would destroy the morals of the White race.
Correct. The term for it is miscegenation, and it was against the law in many states until the Civil Rights era, with laws against it being finally struck down in 1967. It's born out of a concern to defend the "white race."