My poodle developed the same thing and I asked the vet she said that it was just the pigmentation. Kind of like wnat they call pregnancy mask in women. Mine was related to her allergies. Nothing to worry about just kind of ugly.
I agree it is most likely pigmentation I am willing to wager you have a Fawn Boxer with some white markings and the black you see along the edge of the white where it sttarts to turn to fawn. mostly on her chest and underside. Its not allergies its simple ageing. Boxers dont have thick coats that is why you can see these pigment changes in the summer if it is very warm and you spend much time outside you might see more "pink skin" on her chest ,chiin and underside this is a very harmless form of sun burn watch it do not let it weep or turn dark or bright red then you have a serious sun burn and a Vet. trip will be advised Be aware BOXER ARE MORE PRONE TO CANCER, THEN ANY OTHER BREED OF DOG!
I just took my maltese to the vet for something that sounds exactly like what you describe. She said ticks had given her an infectiion and she's on antibiotics now.