Good question. The guy's arm in the background is blue too. All I can think of is the old Polaroid b/w prints. You had to coat the photo surface with a sealer after it dried, otherwise the emulsion could change over time (from light exposure). Maybe that is the reason. Or, if the photo is a b/w enlargement from a negative in a home darkroom, they didn't get fixer on the whole photo. In b/w photography, fixer is usually a sodium thiosulphate solution. My best guesses.