Be firm in checking this out. My vets brushed it off and I went back numerous times. If you think the discoloration looks like brusing, and they seem apathetic, see a blood specialist, and/or use the term vasculitis. I had never heard of it before. Also look for for large patches of grayish color to the skin.
Probably it is just pigmentation. Dog's skin is not just pink. They frequeintly have dark freckes and the pigmentation changes with their body chemistry.