That is -- quite possibly -- not a blister, but an abscess.
Yes. Abscesses will often clear up on their own, but in the mouth region, there's a serious danger of tooth loss.
You can call around and see if there's some sort of low/ no cost vet care in your area. Depending on what government programs you're on, you'd be eligible.
Maxx had that two years ago... found out it was a co-morbidity from his bug bites on his ears. A shot from the vet to boost his immune system proved to work just fine. Now a dab of neem oil on my fingers, a rug or two to each ear and no more bug bites or "canker sore". Ever had one in your mouth when you've gotten really sick? It hurts like heck and really hard to eat. Put some canker sore meds on a q-tip and it will numb it for ya so food will be appealing again... Hope it makes it better for ya!
It could be an abscess or it could be plant poisoning. Do you have plants at home? My Dillan scared me once when he ate plants from the fish tank. His mouth had blisters all around. Poor thing. It went away on its own though, but you definitely want the vet to take a look.
If it is an abscess, your cat definitely needs to see someone and probably get antibiotics.
The last thing you want to do is cover it with anything because that could lead to an anerobic infection.