As a recognized historian, Robertson provides sources in all his book. The reference to C R Mason's black pioneers found on page 427 of Stonewall Jackson: The Man, The Soldier, The Legend comes from R L Dabney and a book written by Mary Anna Jackson, Memoirs of Stonewall Jackson, p 279. If anyone has access to this book, the exact quote would be appreciated.

Here's a markers for the bridge-builders --
http://www.hmdb.org/marker.asp?marker=14080

This incident took place in early June 1862. Describing another incident which tool place later that same month, J. William Jones, Reminiscences Of The Army Of Northern Virginia, mentions Capt Mason as a bridge-builder in a chapter titled, "Seven Days Around Richmond".

General Jackson was delayed by the necessity of rebuilding Grapevine bridge over the Chickahominy, and did not put his column in motion until "early dawn" of the 29th. It was on this occasion that the incident occurred in which figured Captain C.R. Mason -- widely known in Virginia as "the Napoleon of railroad contractors" -- whom Jackson had attached to his staff as chief of pioneers. Anxious to build the bridge and join in the pursuit of the enemy, Jackson sent for Mason, told him his wishes, and ordered him to be ready to begin the bridge, "so soon as the engineers could prepare the plan and specifications." The veteran bridge builder at once replied: "Never mind the pictures, General! If you will just send me men enough who will wade in the water and tote poles, I will have the bridge ready by the time the engineers can prepare the pictures." Jackson cordially seconded his efforts, the bridge was ready in a marvelously short time, and the "foot cavalry " were again on the road.

At this time Mason had no one under his direct command. To fill the immediate need, troops had to be assigned from nearby regiments. Where were his black pioneers?

Coming to Staunton about 1854 to direct work on the Virginia Central Railroad, Mason appears on the 1860 census of Augusta County VA as a railroad superintendent and mail contractor. He and his family lived in a house which still stands on East Beverly Street. According to the census of 1850 and 1860, Mason was born in New York, a slaveholder and about fifty years old. According to his grave marker at Thornrose Cemetery, Mason was a least ten years older (see below). More about Mason --
http://www.roanetnhistory.org/bookread.php?loc=WaddellsAnnals&pgid=518

Without further information, it appears that Mason must have served General Jackson as a civilian volunteer. In the summer of 1861 Mason raised a company called the "Staunton Pioneers" which became Co "H", 52nd Virginia Regiment. Mason resigned his commission in September 1861 to resume his work as a mail contractor and construction engineer. Evidently Mason has a service file as Captain and Assistant Quartermaster which I've been unable to access. If anyone does, please let me know.

Filed under the name 'Mason, Chas R' (an error, his full name being Claiborne Rice Mason), requisitions for personal expenses incurred in 1861-62 are available in fold3. Seventy-one images can be found in this file. Requisitions appear to be for construction and mail contract work, mostly the former. Here's another marker specifically about Mason's work for the Confederate government --
http://www.hmdb.org/marker.asp?marker=15145

Mason's requisition for labor from Apr 1 through June 30, 1862 can be found on image 53. He received a dollar a day for 53 slaves hired for 19 days from Apr 1-19, 1862 ($1007), 46 slaves hired for 64 days from Apr 20-June 22, 1862 ($2944), and 39 slaves hired for 8 days from June 23-30, 1862 ($312). Image 61 represents a requisition for six months hire of seven slaves dated July 1, 1862. These seven are given first and last names, but are specified as slaves by Mason.

Based on Capt Mason's requisition, the men who worked on the bridge over South River near Port Republic VA were slaves. The 53 men hired by Mason in June 1862 were civilians and must have been contracted locally. They may not have worked beyond the confines of the Shenandoah Valley. J William Jones in the account cited above makes it clear that during the Seven Days Battles Mason had no ready access to labor, slave or otherwise.

I find no evidence that Mason accompanied Jackson's command or served him in any capacity after July 1862. There is a break in his requisitions from late July 1862 through the end of September 1862. At this time they resume for construction work. For that reason we cannot assume that Mason's slave labor could have been observed among Confederate troops in Maryland. Slaves were frequently hired in quantity by Confederate quartermasters, but ordinarily labor gangs did not march or camp with the army during field service.

Edward, on another note, if there the references you mentioned really are in the OR, please provide volume and page source.