What is the dark spot visible below the cockpit on A-10s?
It's a false cockpit, a type of camouflage patented in 1980 by Keith Ferris, a US artist and camouflage designer. From some angles, it makes it difficult to determine the orientation of the aircraft. The Canadians were the first to apply it; pictured is a CF-18 Hornet with one:



Notice how at a glance, it takes a second to realize the Hornet is inverted and pulling towards the ground. During dogfights this can be enough to make opposing pilots think the aircraft is going a different direction. It might not seem like much in a photo, or if you're watching an aircraft fly past at an air show or airport. In the stress of combat while pulling extra Gs it's more than enough to confuse or delay a reaction.

For the air to ground mission, this is particularly important. If an A-10 encountered antiaircraft fire or an enemy aircraft it would have to rely on its own agility to escape or gain the upper hand. Other nations have adopted this technique — I've seen French and Russian types, and possibly Gripens of some air force. As far as I know, the A-10 is the only US aircraft regularly camouflaged in this way.