Okay this seems extreme, but the fact that she sucks so hard she blisters her lip worries me too. This is for after you've tried positive reinforcement, the other suggestions, and her doing it on her own.
Ask your dentist if she can make her a palatal crib appliance. the dentist places stainless steel bands around 2 top teeth (molars) and takes an impression of her mouth. Then about two weeks later, it comes back from the lab and they cement it in place. It has a bar that coveres the roof of her mouth, so even if she puts her thumb in, she'll get no suction. Now, the first night is the hardest (REALLY HARD). that thumb is comfort to her and when she can't suck on it, it's very scary and she'll cry ALL NIGHT (be prepared) After one month, the habit should be broken, and the dentist takes out the appliance. it does no damage to her teeth. In fact, it helps because it will prevent thumb sucking deformities and braces later on. When the appliance comes off, make sure she knows that if you catch her sucking her thumb even once, it goes in again for another month. This way, the thing is always in her mouth and always working, since you can't watch her 24/7. when she first gets it in, her mouth will salivate a lot and she'll talk like daffy duck for a few days, but in the long run, it's not that bad. your dental insurance should cover about half the expense and believe me, it's way cheaper than braces. I have a five year old who went thru this a year ago, and is so proud now that she doesn't suck her thumb anymore--it was worth that first night of crying hell.