First, here's an account of Negroes in Illinois service during the war. It states that 1,811 black men served in Illinois commands during the war, the largest number coming from the 29th U.S.C.T., an Illinois Regiment. Evidently a small number were regularly enrolled early in the war (1861-62), the greater number coming after emancipation. The author also notes white resistance to black enrollment. Evidently the response to black men wasn't "Ya'll come" north or south of the Mason-Dixon line.
http://dig.lib.niu.edu/ISHS/ishs-1963autumn/ishs-1963autumn-529.pdf

General Schenk's ammended report (which you cited) is based exclusively on the report of a "perfectly reliable Union man" from Vienna. It would seem that neither Schenk nor any of his soldiers laid eyes on the 150 "armed picked negroes" who lay "unobserved in a grain field" and "did not fire a gun." It is interesting that the Confederate commander in the action at Vienna, June 17, 1861, Col. Maxcy Gregg, makes no mention of "armed picked negroes" in giving credit for victory in this fight. Perhaps this group of 150 was "unobserved" by the Confederates as well!

The War of the Rebellion: a Compilation of the Official Records of the Union and Confederate Armies; Series 1, Volume 2, pp. 127-130.