But uh-oh. I could possibly give you an answer and then discover that the text in the book contradicts me. Did I ever describe Umber putting sugar in his coffee at any point in the trilogy? Not sure. It's been a couple of years since the writing.
Here's what I think: Sugar and milk when he can get it. Black when he can't, for example when he's on the leviathan ship and refrigeration is a problem.