It would be realistic, as well as add fun later on when jets are available, if the sky were to become darker above you the higher you fly.

some video examples of high altitudes:

http://www.youtube.com/watch?v=hKblUBntxRE <Everest summit

http://www.youtube.com/watch?v=NcgLH8CFEZM <Mig-29 flying high

" At higher altitudes, the sky trends to darker colors, since scattering is reduced due to lower air density; an extreme example is the moon, where there is no atmosphere and no scattering, making the sky on the moon black even when the sun is visible." - http://en.wikipedia.org/wiki/Sky