FWIW, figuring that nickel is tough stuff, I kept my nickel S&W Model 38 in a Kramer horsehide pocket holster for maybe a few weeks. As a result, the nickel finish was tarnished in a pattern that matches the texture of the leather. Not real damage, but not attractive, either. After that, I only kept the gun in the holster while I was carrying it, and I wiped the gun with a silicone cloth between uses. The tarnishing continued, although much more slowly.
I don't expect that this holster would exactly tarnish blueing. But I do think that leather, being an organic product, contains enough water and other compounds to promote rust.
BTW, trying to polish the tarnish off my Model 38, I discovered that Flitz, which is not supposed to be abrasive, scratches nickel. Again, not a big deal, except I like to keep my guns in as good a condition as practical.