
This one is going to hurt you, with a few caveats: if your seeker friends help you find it, you can trigger it to do very little horror; if a fellow investigator is trying to use the grimoire, it could turn your tokens right back into blessings; you can use rite of sanctification to keep those blesses out of the bag.
Update: it seems the blessing tokens can be easily sealed. So this weakness is to prevent Mary from just flooding the bag and seeing what happens. As a weakness, it’s pretty interesting how it requires you to work with a rhythm, just like Thrice-Damned Curiosity.
Beat this with deny existence. (Thanks Death by Chocolate.)