Players generally won’t have time to put on glasses between the announcement of Blurry Beeble and you paying its costs. They must have had glasses on already in order to later block it.

If Blurry Beeble was put onto the battlefield without being cast, it can’t be blocked.

Any glasses with lenses will count, prescription or not. This includes sunglasses, but doesn’t include monocles or contacts.

The glasses don’t necessarily have to be on a player’s face for that player to be wearing them. Hanging around your neck, up on your head, or sitting on your leg to make some adorable little denim monster all count.

You can’t remove your opponent’s glasses before casting Blurry Beeble, although you may ask them to remove their glasses. They may fall for it.