And in all your examples that service was, without exception, declined by the government. Government sanction of free blacks in the military was not granted until 1862 (for musicians only) or 1864 (for service positions only) or March 1865 (when blacks were accepted for combat positions.) Unofficially blacks, free and slave, were brought along with the army as cooks, servants, laborers, and the like from the very first.
Aside from the example I quoted who had more help than he could use, would you please cite documentation that says their offers to help build fortifications, etc., were refused.
While we're on this topic, here were a couple of other examples I found in the old newspapers.
COLORED TROOOPS IN THE SOUTH -- Fifteen hundred free colored men in New Orleans, at a meeting last Monday night, enrolled themselves for military duty in defense of the Confederate States. [Philadephia Public Ledger, May 1, 1861]
Baltimorians complain greatly at the want of telegraph and mail facilities. Three or four hundred negroes offered services to Southern Regiments. [Kennebec(Maine) Weekly Journal, April 26, 1861]
Also IIRC, free blacks made up some of the fire crews that put out fires in Charleston, SC, caused by Yankee shelling of civilians.