A blister is a painful problem on any body part, but the tongue would an especially uncomfortable location. Before discussing what to do about the blister, it is important to discuss where it came from and how long you have had it. If this is something that has been present for a long time, or if you are not sure about where the blister came from, then you should see a doctor
to make sure that it is not something more serious.
While it would be unlikely for most patients, some malignancies can present in this way, especially if you have a family history of cancer, smoke tobacco, or drink alcohol. If the blister has just been there for a short time, then it could possibly be a canker sore, which is very common. These are painful, but generally resolve quickly. There are multiple treatment options available, some of them over the counter. Simple local anesthetics can be applied with good results as well.
Please speak to your doctor to make sure that you treat the blister appropriately and do not simply cover the symptoms with local pain killers, however.