On July 14, 2015 NASA’s New Horizons probe captured Pluto’s night-side illuminated by sunlight. Ponder blue skies of Pluto – not so different from planet Earth.
Blue sky phenomenon stems from small atmospheric particles scattering sunlight. Earthly particles are nitrogen molecules, on Pluto sootish little particles called tholins do the job. From spaceweather.com –
“The term “tholin” was coined by Carl Sagan and Bishun Khare to describe organic substances they obtained in Miller-Urey experiments on gas mixtures akin to atmosphere of Saturn’s moon Titan. On Pluto, tholins form high in the atmosphere where UV sunlight breaks apart nitrogen and methane molecules. The fragments re-combine to form complex macromolecules. These macromolecules continue to combine and grow until they become “tholins.”
Nightside of Earth – http://jesper-ullbing.deviantart.com/art/Nightside-Of-Earth-190372739
Tholins themselves aren’t blue, rather gray or red particles scattering sunlight. Science believes some of Pluto’s splotchy red surface comes from Tholin particles falling from the atmosphere.