Altitude definitely has something to do with the bluer blue skies. As the atmosphere gets thinner, the sky appears darker, resulting in that bluer blue. The next time you're on an airliner up in the flight levels (FL300 and higher), take a look out the window and notice the difference in the blue from the bottom of your window to the top. Looking up, that sky looks very different than it does at sea-level. If you think that's dark, take a look at photos taken by U-2 pilots (or SR-71 Sled Drivers, back in the day) above FL600. The sky above them is basically black, like space.
I have noted that photos taken on the Colorado Narrow Gauge definitely depict deeper blue skies than what you'd see back east here in New England.
/Kevin Madore
Edited 1 time(s). Last edit at 10/19/2016 09:31AM by KevinM.