I am sure you have done your research and are well informed about genital blisters
being associated with herpes
As you have not been sexually active for quite sometime chances of this being herpes are slim, but it cannot be ruled out.
There have been women who present with genital blisters or sores at times purely due to friction due to the genital region being constantly exposed to friction.
I would like to first say that you need to avoid touching the blister or irritating it. Wear comfortable undergarments or place a sterile pad in place, to avoid further friction or the blister blasting that can result in it getting infected.
Please visit your doctor, but in the meantime avoid friction by keeping your genital region clean and dry and wear comfortable undergarments.
Hope this helps.