Can thrush in the mouth cause tiny blood blisters if not what causes them?

Unlikely: Are you sure they are small blisters and not just large taste buds? A lot of people never really look at their tongues well. Candida (thrush) really shouldn't do this unless its very very severe.

Unusual: It is my policy never to say never, but this would be unusual. If you are an adult with thrush, something may be seriously wrong. The most frequent thing that infectious diseases doctors think of is HIV disease, but there can be other causes. Strongly advise you to see a doctor and be thoroughly evaluated, including tests of your immune function.