When a magnetic tube pops through the surface, it interrupts heat flow from below and drops the local temperature to about 4,500 K, cooler than the 5,700 K temperature of the surrounding photosphere. That's what makes a sunspot dark: it's still hot, but it's cooler than the rest of the photosphere.It still emits light, just not as much as its surroundings. The coolest and darkest area of a sunspot is the umbra; the slightly hotter and brighter area surrounding the umbra is the penumbra.

Some larger sunspots are visible without a telescope (but still with a proper filter). In fact, Chinese astronomers were the first to record the observation of these spots in 28 B.C., probably when the Sun was low on the horizon and atmospheric dust dimmed the Sun enough to see without a filter. Legend has it that a large spot was also seen in 813 A.D. upon the death of Charlemagne.

SAFETY NOTE: Don't try this method yourself, that is, looking at the Sun without a filter when it is low and dim on the horizon before sunset or after sunrise. It's too risky. If you have to squint, then it's too bright.