and lost with his "niggers."(52)
Racism against the black infantrymen was rampant. Lowell puts the word in quotation marks to make sure everyone knows it's not his word, but borrowed from the racist haters during the Civil War era. That they were thrown in a ditch all together gives you a sense of how little respect their enemies had for them. That is not exactly the picture of sportsmanship.
the drained faces of Negro school-children rise like balloons (60).
The faces of the children reflect African-Americans' struggle since slavery. Although they're rising (seemingly a good thing), Lowell writes that their faces look drained (presumably tired from the centuries-long fight for civil rights). This is also perhaps one of the weirdest images in the entire poem. It takes a certain amount of cartoon-watching imagination to be able to picture this one.
a savage servility (67)
Though slavery is long over, some of the same awful qualities are still alive in present-day Boston. Lowell might also be playing with the word "savage." A common racist idea of the Civil War era was that African-Americans were like savages. In mashing together "savage" and "servility," he calls to mind the perfect stereotype of a slave—long after the presumed end of slavery (and the backward thinking that excused that awful practice).