I have a couple of rather large blisters under my PEI sheet
I was planning on puncturing them tonight with a small hyperdemic needle to release the air. I have seen reports that the hole you puncture will heal after a few prints and the blisters will re-appear. I was thinking of using the need to inject something under there to remove the space for the air after all it is the air expanding that causes the bubbles.
I am thinking thermal paste as it doesn't expand much with heat, so if partially fill the blister when hot, when it contracts it will squeeze out the extra paste leaving it flat. My other option was super glue, but that seems like a pain in the ass.