Sunspots looks black on region of sun that is photosphere. They are relatively cooler areas compare to remaining surface. There temperature can be 1500 Kelvin lower than photosphere that is above 5500 Kelvin. How they are formed? They are result of high magnetic field which is not fully understood yet. High gas jet shootup and coronal mass gets emits from here. Magnetic fields affect out earth communication too which can be a big deal thus various space agencies are conductiong missions.
Here, you get to know about what is sunspots. To know more about sun atmosphere clickhere.