Me, personally, i'd never boil or heat the mussel to open them. Just shell them, (with a little practice and a decent knife you can do it qute quickly) and freeze.
I blanched some mussels once, they did get toughter, but the scent was gone and they were not a very good bait.
Shell fresh mussels and freeze in ziplock bags . . i collect a couple of buckets every summer Aug/Sept and freeze 'em down for the winter . .
Each to their own i guess . .just my 2c worth!
