You don't want anything to seep inside the case and contaminate the powder (in the case of blanks, there may not be any powder - it depends). But the same things goes for the primers - they can get contaminated too. Then your blanks will have a new name. "Duds". I would just store them in a sealer waterproof container (Pelican box or something) if you have need for reliably operating blanks in a wet environment.