I am not a mom, but I have seen a lot of diaper rash in my days, between being a daycare director and a nurse..... lots of babies butts have come my way...
I am a little concerned about the blisters... More and More I have seen little babies and kids coming to the emergency room for really bad diaper rashes that have turned out to be skin MRSA infections... I would only worry about it if your child goes to daycare because sometimes they are not real good about washing the diaper changing areas.
If it is persistant and hasn't been checked by a doctor at least once, I would recommend doing that just to be sure, like Melissa said to see if it is fungal or a bacteria type thing.. IF not.. I recommend BUTT Paste.... It works wonderful and has three or four different ingredients in it to make a good barrier...
Just make sure before you make the barrier that you are not sealing in an infection of some sort.. Stay away from any scented lotions. Keep the area really clean, and let her butt be exposed to the air as much as possible...