Redness, blisters and itching caused by under wear in a hot area?

Several causes: for what you describe: Generally yeast thrive in hot, sweaty areas, and heat rash/milia can also occur there, however they do not form blisters. If the area has hair you may have folliculitis, an infection of the hair follicles, or you may have a bacterial infection of the skin (staph and strep can cause blisters), or possibly herpes virus. Best to see a doctor- they can take a culture if needed