Sunspots are areas of the Sun’s photosphere (the visible surface) that are significantly cooler than the surrounding regions. Although the exact details of sunspot formation are not fully understood, they are coincident with areas of increased magnetic field.
These intense magnetic fields appear to suppress the release of heat into the photosphere, thus making sunspots cooler than their surroundings by a couple of thousand degrees Celsius. This means that sunspots are only about a third as bright as the surrounding photosphere, and it’s this contrast in brightness that makes them appear dark, even black.
If you could pluck a sunspot from the Sun and put it in the night sky, it would actually be about as bright as the surface of the Moon as seen from Earth.