hi and welcome, I hope we can help you and kitty out with some answers. allergies to diet can cause many symptoms. I will give you a site with some pictures for you to compare, kitties blister could well be ECG or eosinophilic granuloma complex....indolent ulcer.
look at that particular photo and see if you think it compares to your kitties and read what the site has to say than we'll discuss further okay.
www.ivis.org/proceedings/hills/felinemed/hill1.pdf
here is another site for you to look at indolent ulcers of the lip.....
these sites give good information however I do NOT agree with the steroid therapy...this can lead to diabetes in cats and/or many other diseases.....there are other ways to treat more holistically...
http://www.marvistavet.com/html/body_eosinophilic_granuloma.html
Hi, IMO 8 mos is a VERY long time w/ no improvement. I would definitely ask about a biopsy to see what this actually is. If you can, see a veterinary dermatologist. You and your kitty need some answers. This biopsy will give you the answers you need, and that way you'll be able to give the correct meds that will actually help the problem. Prednisone, is a "band aid" when the vet doesn't know what's wrong and not good for your cat long term. There are many dangerous side effects/problems this drug can cause long-term.
I hope you get the answers that you and your kitty deserve! Wishing you well!
ZQ
I'd suggest eusinophilic granuloma, as well. It can be tested and if found to be full of eusinophiles, you have your answer.