Hi
As hand-foot-mouth (HFM) disease explicitly states, palms, soles & oral cavity are affected in this viral disease.
Blisters on buttocks are unlikely to be related to HFM, but then nothing is impossible in medical science too!
Next Steps
Unless blister is infected, or child is immunocompromised, no need to aggressively treat: can give short trial of topical antibiotic (like fucidin) if secondary bacterial infection & watch closely for response
Health Tips
Keep blister area dry