What can i do to stop the bleeding ulcer / blister in my mouth?

First things first: You need to have this lesion examined by a professional. It must be determined of it is viral or bacterial in nature, or if a biopsy is indicated. Make an appointment with an oral surgeon to have it examined.

Rinse: Dilute salt water rinse or gargle, any persistant open sore should be evaluated or biopsied.

Saline wash and spit: You have a mouth ulcer and it bleeds. Try using a salt water wash. 1 teaspoon in a cup of water dissolved. Wash 4 times a day and spit it out. If you get these ulcers a lot that is unusual and you should let your doctor know. If the bleeding is anything more than a small amount you should also let your doctor know. A history of any smoking or oral tobacco use is important.