Stucco blisters are caused by water damage. While stucco can get wet (it’s an exterior surface after all), it doesn’t like to get wet often or stay wet for long periods of time. One of the most common reasons for moisture damage is improper drainage. Gutters that leak water against the side of the house or downspouts that allow a lot of splashing water to hit the walls are very common.
There are other ways water can damage your stucco. Your best bet at this point is to have a licensed contractor pay a visit to your house. A quick inspection should give them all the information they need to diagnose your problem. I’d come out myself, but I’m in Arizona!