Answer: because they are cooler than their surroundings
A sunspot is a region of the Sun that has a lower temperature than its surroundings, and with an intense magnetic activity. This is due to disturbances in the Sun's magnetic field that emanates towards its photosphere.
In this sense, sunspots are darker because they are cooler than their surroundings. However, they are dark only in contrast to the bright face of the Sun.
