I get a blood blister on my tongue for no reason it's stays for about half an hour then disappears why does this happen?

Tongue blister: I believe what you have described is a mucocele. A salivary duct is plugged or otherwise not functioning properly. Every time you increase the salivary flow, the gland and duct swell up and finally burst. Salivary glands are of a great variety of sizes. If this is a very minor gland it remains a minor annoyance. You should talk with your dentist. You may have to have it removed.

Sure? Blood Blister: If you are sure it's a blood blister: thinning of a blood vessel wall could cause "ballooning" when the pressure goes up, then collapse when the pressure goes back down. The concern is if this blister doesn't resolve and gets cut, getting it to stop bleeding could be difficult. Have it checked out with an oral surgeon.