Moisture has gotten behind the area and has frozen, then thawed giving you the bubble. The stucco is a three layer process. A metal lath is applied to the sheathing, rough mortar is applied, then a finish mortar layer is applied and "brushed" out exposing some fine aggregate, then painted.
If the wall is reasonably dry - I would try using a product called LEVEL-LASTIC. It's mostl;y used for levelling floors but should work on walls - can be trowelled to an eigth inch thick with no shrinking.