Everything I have read about jewelry smudges points to the jewelry itself.
Whether it be on skin or clothes, the jewelry item itself, or rather the
alloy, is what leaves the black stuff. Acids in the body, body powders,
clothing. All will, can, and do make jewelry leave that black smudge.
Rub a clean piece of jewelry on a clean piece of cloth and you have a
smudge. Works the same way when wearing the jewelry, only much slower.
The heavier and larger the piece of jewelry, the faster the smudge
appears. Not much one can do about it.