I have a blister inside my mouth! Is this dangerous?

Determine cause. With so little info to go on, it is impossible to tell what that lesion represents... Where is it located? How long is it there? What are your smoking and drinking habits? These questions need to be answered, along with a visualization of the problem. Start with a local dentist to rule out a dental abscess, then see your physician.
It can be. You may have a simple canker sore or a herpes viral lesion make sure that you avoid kissing someone while it is present avoid any oral contact if the problem persists see a periodontist.