A sunspot is a region on the Sun's surface (photosphere) that is marked by a lower temperature than its surroundings and intense magnetic activity, which inhibits convection, forming areas of low surface temperature.
Although they are blindingly bright at temperatures of roughly 4000-4500 K, the contrast with the surrounding material at some 5700 K leaves them clearly visible as dark spots.
If they were isolated from the surrounding photosphere they would be brighter than an electric arc.
Sunspot numbers have been recorded since 1700 AD and estimated back to 11,000 BP.
The recent trend is upward from 1900 to the 1960s, then somewhat downward.
The Sun was last similarly active over 8,000 years ago.