Hello.Thanks for writing in .
If you have seen the itchy blister appear suddenly, it might be due to any contact dermatitis
resulting from insect secretion .YOu are right in applying a cortisone
cream ; for the itching, you may take an anti hisamne tablet like Allegra or Hydroxine or Cetrizine
. You should continue it till you have the itching /burning.
If for symptoms do not improve with this in the next 3-4 days, please visit a dermatologist
to do the needful.
hope this helps..TAKE CARE!