what is your age? blood blisters may be from some hematological disease. may occur due to some clotting factor deficiency. anyway blood cancer
may also has similar signs. but not to be worried too much, because detailed history and clinical exam is mandatory. no of lesion, size, appearance,margin everything to be looked at. consult a hematologist
probably a complete blood count
to be done then if indicated other exam like bone marrow
biopsy may necessary.