Why does the sky look black in pictures taken from the summit of everest?

However, this same black sky effect does not occur when looking out the window of a plane.

It doesn't?

(image credit: http://www.123rf.com/photo_10994787_view-of-jet-plane-wing.html)

Don't forget that the bulk of the atmosphere's mass is in the troposphere and it is not uniform in thickness around the globe. In fact the troposphere is almost twice as thick around the equator as it is at the poles. This is due to differing air density resulting from substantially different average air temperature close to the surface.

Consequently, when flying in an airliner at say 10km altitude in equatorial or mid latitudes, your aircraft is still well and truly in the troposphere with plenty of atmosphere still above you. However flying at latitudes nearer the north or south pole at the same 10km altitude, you will be flying very close to the top of, or even above the troposphere.

Therefore despite being at the same altitude, the sky will look significantly darker for a passenger flying in the middle of the day at high latitudes than someone at mid or equatorial latitudes.

Hope this helps resolve some of the confusion regarding peoples differing opinions on how dark the sky looks from an airliner.