In pictures taken from the summit of Mount Everest (such as this one), the colour of the sky is a very dark blue or even black in some pictures. I remember from my own experiences of hiking in the Himalayas that the sky appeared to be darker blue as I climbed higher. In shots looking outward from the summit there's a very interesting effect of having a black sky above with a blue sky lower down near the horizon. (one example here) I believed that this happens because there is less atmosphere above the climbers to scatter the blue wavelengths.
However, this same black sky effect does not occur when looking out the window of a plane. As the plane climbs, the sky does not appear to "change colour" at all as the plane ascends. The cruising altitude of commercial airliners on an international flight is slightly higher than Everest, so one would expect the sky to appear quite similar.
What explains these two observations?