The buttock area, especially the top part is prone for being moist due to sweat and sebum. Blister occurring could be fungal infection
called tenia or ring worm infection
which is very common in that region. If blisters are containing fluid could be bacterial infection also. Good personal hygeine is must for this condition. Local application of antifungal, antibacterial cream is necessary. If recurring many times, then consult your dermatologist
for antifungal drugs treatment.