Staining from metal

Hi all

I made a few of these secateurs holders a couple of weeks ago for myself and some arborist friends. I went to use them today and saw this blue staining; obviously from coming into contact with the steel of the secateurs. In my ignorance a

s a newbie leatherworker I assumed that it was only chrome-tanned leather that reacted to metal, not veg-tan. It looks like I was incredibly naive and wrong. There's nothing to be done about it now (I guess), but what can I do in future to prevent this happening again? I've been asked to make a couple of axe sheathes and I certainly don't want a repeat of this fatal error! Any help and guidance would be thoroughly appreciated. Thank you.