Hello. Thank you for writing to us at healthcaremagic
Longer the sexual act carries on, higher are the chances of contracting an STD from an infected sexual partner.
Having said that and no matter how brief, it is unsafe to have an unprotected sexual encounter with a sexual parter of unknown sexual status.
There is nothing really that you can do now, to reduce the risk. The best you did was to stop immediately you noticed a
blister.
Performing
Oral sex on a male partner carries a risk for various STD's like herpes labialis,
syphilis,
gonorrhea, chlamydia. HIV is a risk if either of the sexual partners engaged in oral sex has active sores/ ulcers.
Presence of Sores increases chances of HIV transmission because some amount of blood cells would be present in the secretions thus increasing the risk.
I suggest you to visit a sexual health counsellor as well as undergo screening test for HIV, syphilis,
Genital Herpes, gonorrhea, chlamydia at appropriate intervals.
Regards