New Masters Descend from New England
Northern troops did not liberate black slaves as much as they denied their valuable labor to Southern planters, and then re-employed them under new masters as the British had done in the Revolution and War of 1812.  The British called them “Pioneer Corps” and set them to hard labor on fortifications, the blue-clad New Englanders called them “labor battalions” and set them to hard labor on fortifications.  Prior to the Northern invasion, slaves were well-fed and given medical treatment, but were yet to suffer blanket-tossing at the hands of the lusty descendants of New England slave traders.
Bernhard Thuersam, Chairman
North Carolina War Between the States Sesquicentennial Commission
"The Official Website of the North Carolina WBTS Sesquicentennial"
New Masters Descend from New England:
[In the spring of 1862, Northern General John G. Foster was] in command of a mere 9,500 troops – scarcely enough for defensive security, and far below the number needed to conduct significant offensive operations. To maximize the effectiveness of his troops, Foster set to work, turning New Bern [North Carolina] into an impregnable stronghold.
Much of the labor was done by hundreds of “contraband” Negroes. Foster organized the ex-slaves into quasi-military formations, gave them a fife and drum, had a special flag created for them, and saw to it they were well-fed and given medical treatment.  In return for this unusual policy, the labor battalions worked diligently and in high spirits, fortifying, as it were, their own newly gained freedom.
The Yankee soldiers felt secure behind Foster’s earthworks…lived in decent quarters, ate good food, worked reasonable hours, suffered less disease…and had ample opportunity for recreation.  Their recreation ranged from concerts to literary clubs, with an outside chance of enjoying female companionship now and again.
Throughout the surviving letters and regimental histories, the attitude of the occupiers toward the Negro population seems rather contradictory. On the one hand, the Yankees enjoyed playing the liberators….On the other hand, they were quite eager to obtain for themselves the services of a low-paid “pickaninny” servant, and apparently saw no contradiction. Negroes were also a constant source of wonder and bemusement, which tended to express itself in utter condescension.  They are often described in minstrel-show terms, complete with the eye-rolling darkie stereotypes, and their keen, completely natural, interest in their new masters prompted the soldiers to think of the Negroes as “the novelty-loving sons of Africa.”
Sometimes the Negroes were nothing more than the butt of amusement for bored soldiers, as this excerpt from the official history of the 24th Massachusetts indicates:
“Late in the afternoon, the regiment was mustered for pay, and later still the men had a spell of that almost universal horse-play known to those days as “tossing niggers” in a blanket. “While it was fun for the tossers, and very likely did not hurt those tossed, it was for the latter a period of intense fear, not to say horror.  Of course the poor victims screamed and yelled, but the louder the cries, the greater the fun for the lusty fellows at the blanket’s edge….”
(Ironclads and Columbiads, William R. Trotter, John F. Blair, Publisher, 1989, pp. 157-160)