A blister cave is a cave formed by gas pressure pushing up a surface crust.
It is obviously hard for gas to modify rock just by pressure. So the geologic situations where this happens are rather rare. The most common one is a calm volcanic eruption. Gases coming out of the lava itself, mostly carbon dioxide and steam, form bubbles inside the still liquid lava. The more tenacious the lava already is, the bigger the bubbles can become. The the lava cooles down and the blister becomes a cave.
Blister caves are primary caves, as they are formed together with the surrounding rocks. They are also lava caves, as they are formed in lava. They are much smaller than lava tubes, although they may have a rather big diameter, because they always consist of a single blister.
True blister caves are very rare, generally the blisters are too small to enter. The extreme low number of known blister caves is probably because they are not recognized as such. Some may be mistaken as a short piece of a lava tube. And their live span is rather short, as the lava begins to erode as soon as it cools down. Similar to lava tubes, they are generally completely destroyed after 20,000 years, which means there are no caves remaining in older lava streams.
The term has been misused for single chambers formed by other processes. Most commonly the drainage of liquid lava from beneath, so called stony rises, produces rather similar caves. It is very hard to determine the difference.
The biggest blister caves are found at Mt. Fantalo, Ethiopia. Some 130 blisters are known, where the still intact blisters are up to 30 m in diameter, but the already collapsed ones are up to 100 m.
There is on one blister cave listed on showcaves.com, Grotte du Chien in France. As far as we know it is the only blister cave which is a show cave.