Welcome to HCM.
It does not seem like a herpes blister
because herpes usually causes a group of blisters which is fluid filled and itchy. Genital herpes
usually is sexually transmitted.
the herpes which is brought about by sickness is herpes simplex
1 which affects lips and mouth. I suggest application of antibacterial cream on the blister for a week. If it does not reduce get tin touch with your GP and get it examined. Thanks.