Hi, my son got them and they were horribly frightening to look at, he had them all over. The family doc looked at them and called them apthous ulcers (canker sores)even with the blood inside them, and gave him steroid dental cream (dental triamcinolone) to put on them, and the old miracle mouthwash. It took him a good month to get rid of them all. MY son's blood runs a little odd and perhaps that explains the blood blister appearance. His cbc is seldom perfectly normal. He has epilepsy and takes medicine.
The family doc or ear nose and throat or rheumie should help. The rheumie migth be the best as there are some other autoimmune diseases like Behcets and another one that tends to occur after an infection (I forget the name of it) and they also produce that type of mouth sore. I also believe some kinds of common infections can do that too.
I would get to the doc and get it treated as they can take a long time to get better and you don't want them to get worse.
I hope you feel better soon. The steroid cream and miracle mouthwash helped my son the most. They may have given him some pill medication too, I don't remember.
Ills--Sjogrens-Lupus cond., AI polygland. dysfunction 2, hyper/hypopigment, scoliosis,kyphosis,stenosis, deg.,O.A.,spine surgeries, salivary/lymphectomies, NASH, COPD, RLS, UT/GI bleeds, hystero, brain/nerve damage,TB
Meds--INH,Plaquenil, Evoxac, Metformin, Synthroid, HCTZ, HRT and Lidocaine patchs, Voltaren gel, Klonopin, Vicodin, Restasis, Albuterol, steroids