I'm so sorry that you are having this issue. I have not heard of anyone getting blisters in their bladder.
One thing that seems to soothe my bladder after my BCG treatments is warm green tea.
I buy the powdered green Matcha tea (mix it with hot water) and drink a few cups every day. I've had only 9 BCG treatments, so the inside of my bladder still looks good.
I hope this helps
LillianG
5/2015 4x2cm HG Ta Papillary TCC; 6 initial BCG followed by 3 sets of BCG maintenance. Ten scopes - all NED. Now at scope 1x year.