Tag mystery solved

So Meenah has had this grayish black stain on her chest for some time. Try as we might, we could not figure out what she was getting into to cause this discoloration of her white chest fur.,

All at once it came to me! Her tags, two of the three at least, are made of aluminum. This metal gives off a blackish oxide, so that is probably the culprit.

Her three tags are: One we got with her name and our phone #. Two, her rabies vaccination tag and three is the dog license. Interestingly, only the town license is made of high quality brass.