Thanks for the query,Blister
in mouth after sexual intercourse points towards herpes
Brownish discharge can also be seen in herpes,
You can get a smear done from the blister.
This will help in diagnosing herpes.
Please meet a dermatologist
for final diagnosis and treatment
Let me know if you have any other doubt.
you can ask a direct question to me on this forum, following the below link.
Wishing you a good health.