"Palladio" finish simply means the color is lustrous, silvery-white... the same color as the chemical element Palladium. I don't own a BV keychain but I don't think it will tarnish quickly. Neither will it require constant maintenance or polishing. BV pays special attention to quality and craftsmanship in everything they produce so I doubt that they'll sell something that corrodes after a few uses.
On the other hand, BV also does another type of finish on some of their accessories and bag hardware called "Brunito" which literally means "burnished" (to make dark) in English. Just thought of sharing too since we're on the topic of metal finishes.
Comparing Palladio & Brunito metal finish (on a BV Knot bracelet)
Brunito doesn't necessarily mean dark (like the photo above) ... it also has relatively lighter incarnations like this one below. (brassy, matte-silver-grey vs. the chrome-like palladio)