@Article{ messner1975,
	author = {William F. Messner},
	title = {{The Vicksburg Campaign of 1862: A Case Study in the Federal Utilization of Black Labor}},
	journal = {Louisiana History},
	volume = 16,
	number = 4,
	month = {Autumn},
	pages = {371--381},
	year = 1975,
}

http://www.jstor.org/stable/4231513

Summary

Union operations near Vicksburg in the summer of 1862 marked a transitional moment in the Union’s use of black labor in the Southwest. He covers Williams’s use of contraband laborers to dig a ditch outside of Vicksburg, and his subsequent abandonment of fugitives or forcible return of them to their masters. But then he turns to the use of black labor at the Battle of Baton Rouge, after which contraband were not abandoned but were taken to New Orleans and housed in contraband camps.

Messner appears to have a longer dissertation from Wisconsin, 1972, on interactions between African Americans and the federal army in the Gulf Department.