Black Slaves in Blue and Under Fire
The author below relates that black soldiers were intended for labor duty in the Northern armies rather than fighting for emancipation. If used for offensive operations at all, black soldiers were most often assigned to suicidal assaults which would spare the lives of white Northerners who viewed the futility safely from the rear. At the February, 1865 Forks Road battle near Wilmington, and out of range of supporting gunboat fire, the 6th US Colored Infantry was hurled three times against Gen. Robert F. Hoke’s entrenched North Carolinians, never once getting closer than 150 yards of the breastworks.  The Northern commander would have to await Hoke’s withdrawal before advancing.
Bernhard Thuersam, Chairman
North Carolina War Between the States Sesquicentennial Commission
"The Official Website of the North Carolina WBTS Sesquicentennial"
Black Slaves in Blue and Under Fire:
“About 43 percent of the 6th [US Colored Infantry] Regiment had volunteered for military service. Another 31 percent were drafted, and over one-quarter of the regiment were listed as “substitute.”  A conscriptee could avoid military service if he furnished an able-bodied substitute to take his place. Most substitutes in this regiment were young, usually in their twenties.     A youth might well agree to be a substitute; he might likely be drafted anyway; better to join and accept a substantial cash payment for taking someone’s place.
The soldiers hailed from twenty-three different States, both North and South, as well as the District of Columbia. The most common State of birth was Pennsylvania. Of those whose birthplace is listed, over 36 percent of the men of the 6th Regiment claimed Pennsylvania as their birth place. Delaware and Maryland claimed 16 and 15 percent respectively, and Virginia, another 12 percent. Canada, providing twenty-two soldiers, stood as the most frequent birthplace of any foreign nation.  Like most black units, the 6th Regiment would be assigned to an unusually high amount of physical labor particularly at building fortifications.
From the time blacks had first been recruited it generally had been understood that they were to serve as laborers, and they were used disproportionately often in that role. Their work at Dutch Gap [in Virginia] would have been physically demanding under the best of circumstances, but this assignment included a complication that made it especially difficult and dangerous – they would have to do the [canal digging] work within range of Confederate artillery. 
They burrowed into the steep walls of the canal to make caves for shelter [but the] mortar shells were deadly. They were fired high into the air “and then fell by their own weight, with no warning scream, and, dropping in the midst of busy groups, burst into raged fragments of iron, which maimed and killed.”
Union artillery was brought in to silence those mortars, but its task was nearly impossible….[as] they would try to direct their fire at [mortar positions]…Confederate sharpshooters stationed in hiding near the riverbank would open fire on the artillery crews and distract them from their task.”
(Strike the Blow for Freedom, The 6th US Colored Infantry in the Civil War, James M. Paradis, White Mane Books, 1998, pp. 34-35, 61-64)