What to wear *inside* a blizzard bag

Earplugs!

A Blizzard bag is supposed to be breathable, but I would reserve it's use for emergency only.

The inside of a blizzard bag is perforated. The theory being that moisture will pass through the holes and condense on the inner surface of the outer layer leaving the inner most layer of the blizzard bag dry.

I have not tried this in practice though. I have just purchased one and will be using it this summer as part of an emergency kit for trail running. If it is dry outside I would use any down on the outside of the blizzard bag but if it is raining out the down would go in the bag with me and I would wear it until it was useless. Damp down from humidity still provides decent insulation at a reduced loft. It really needs to be wet in my opinion to be useless.

If you wear a VBL(vapor barrier layer) under your insulation layer, then you won't have any condensation to really worry about. It will all be contained inside your VBL layer, and won't impair the effectiveness of your insulation.
Many people use VBLs for more than just sleeping,for this reason.

This may not be a help in an emergency, but if you are using the blizzard bag for a planned trail shelter and have concerns about wetting-out the insulation layer in a non-breathable bag, and can plan your options in advance, you can couple it with a VBL under your insulation layer.

And even in an emergency kit, there are cuben-fiber VBL clothing systems which are only a few ounces, and could do the job at very low carry weight. They aren't inexpensive, since cuben is not cheap stuff, but they are available. Put the VBL in the emergency kit with the blizzard bag.

Slept in one once with a syntehic fil jacket and I dod not get too damp, just make sure to bring ear plugs as they are a bit crinkley :-)