Blacks worked in civilian occupations such as foundry and arms manufacturing too.
The lion share of blacks were in engineering, medical, supply and cavalry support (blacksmiths etc.). They were a force multiplier.
Force multipliers become exxtraneous the instant the force becomes insufficient to do battle...
Failure to employ musket bearing blacks in the field was but one of several missed opportunities by the Davis adminstration to populate his military...agrarian workers, numbering in the thousands, were virtually exempted from conscription, due to the short-sighted insistence on continuing cash crop production (cotton) so as not to lose the European market...and if you have some documentation stating that blacks took over civil production jobs from whites in any significant numbers I’d like to see it, because this was a manpower drain as well...