Are they the blackish/blueish ones or the green lip New Zealander? Thaw by running under cold water for about 10 minutes. Are they debearded? After the under running water, treat like fresh. Kenji at Serious Eats debunks the "unopened" thing "*For the record, you can go ahead and ignore the conventional wisdom that a mussel that doesn't open after cooking shouldn't be eaten. AsDaniel pointed out in his piece on Rhode Island Clam Chowder (and as corroborated by this Australian study)" I'd do a little test drive with a few for confidence.