It could be just a pigment change...but without seeing it, I would want to make sure that is is not a hemmorage. Cappillaries can break open and cause a deep bruising on the surface of the body. This can be a sign of vasculitis, a blood disorder which can come on a few months after vacinations (for for some other reason). This killed my dog. I don't want to scare you, but you should have it checked out, and keep that info in mind. It can be cured, if caught early, I believe.