They are black because they had to change to their new surroundings and eventually they will change back when their surroundings do as well. Maybe this is a hint about the war and the men who left for it. How when they left for war they had to change to fit their new odd surroundings and then eventually after they get home at some point they will change back to what they used to be, but until they change back they will stick to what they are comfortable with like the less appetizing cold food instead of the food fresh after its been cooked.This was a new way for me to look at this story and even got me thinking about how life works and how events in our lives define us. Thinking about how events change us is especially interesting when applying the idea to war. In what ways are the changes that soldiers go through during war temporary, and in what ways are they permanent?
In "Big Two-Hearted River" I remember Nick being surprised that they are actually completely black, and not covered in something that makes them look black. Maybe this speaks to permanent change when he thought it should be temporary.