Yesterday I read a discussion on dog tags turning the white on their corgis black/grey. I realized this AM that I had had the same problem and you might want to check not the tags but the piece that connects the two.My rabies tags and some other of the ID tags have a cheap piece of metal (not sure what) and that is what turned my dogs neck dark. Once I switched to that piece being stainless steel my problems were gone...finally no more dark spots!!! Of coarse when I suggested to my vet that this piece was the cause and why...he just looked at me when I suggested a higher quality piece and I'm not sure but it probably contains alot of lead too...Jane