I agree David that it's the leather. I had an RB (Reunion Blues) bag before this made of cordura and it didn't tarnish my horn. It was an older RB bag from when Glenn Cronkhite still worked at RB.
I'm not sure if Glenn Cronkhite has made any changes to his leather bag design or not recently. I contacted him about this issue a couple of years ago. I wrote in an email that my bag was bought used so I didn't expect him to do anything about it - just wanted any suggestions he might have. His reply was that he had heard from just one other person (tuba player) who had a tarnish problem. He actually offered to take the bag back and try to add some extra liner inside (what a guy!). But by then I had already figured out the shower cap cover.