In fact, sunspots are one kind of standard stable nuclear fusion reactors...
The common understanding is, temperature of sunspots is too low to mainstain thermonuclear fusion. In fact, this fusion is active only at the relativelly small zone at the center of Sun, from which most of solar neutrinos is generated. IMO sunspots are kind of bubbles in solar plasma, which are raising to the surface, like bubbles are raising to the surface of fluid - so they can exist only limited amount of time, before they burst or dissolve.
Some mushroom clouds of hydrogen bombs are black too, that is, their temperatures are lower than that of their surrounding atmosphere. The temperatures of umbrae of the sunspots are approximate 4,500k, but the temperatures of plasmas in eyewalls of sunspots are certainly above ten million Kelvin.
At present, astronomers think that sunspots are magnetic flux tubes, sunspots cannot be some kind of bubbles, because bubbles cannot produce magnetic fields of sunspots.
