Were you to stand, a watcher of the skies, like Cortez "silent upon a peak in Darien", you might notice that what you saw was not entirely blue; the cloud-free portion of the sky would have a purple tinge, giving an observer a certain inkling of the black. This happens because at high altitudes most of the atmosphere is down below; the air above is thin, and therefore the process of scattering, dealt with at some length in the Weather Eye of yesterday, is less effective.
Let us briefly recall this scattering phenomenon. Sunlight travels very rapidly as a series of tiny waves. The waves vary in size, or "wavelength", and our eyes detect the different wavelengths as different colours. In the case of blue light, the wavelength is very short; red light consists of longer waves; and the wavelengths in between give us the other colours of the spectrum.
The light coming from the sun is white light, a mixture of many colours, and as it passes through the earth's atmosphere, light waves heading in any given direction are deflected from their original path by the molecules of air. This scattering affects some wavelengths more than others; the longer red waves surge straight through, virtually unaffected - and so to a slightly lesser extent do all the other colours - but the blue part of the sunlight is scattered very effectively.
Now imagine yourself looking at the sky on a sunny day. The only light approaching the earth comes from the sun. If you look away from the sun, no light comes towards you from space at that angle; what you see is sunlight which was originally heading somewhere else, but has been "scattered" towards you by the atmosphere. And since only the very short "blue" wavelengths are affected in this way, our eyes see this scattered light - and therefore the sky itself - as blue.
Returning to Cortez upon his peak in Darien, we can observe that because of his high altitude, the atmosphere above his head is very thin. Therefore less scattering takes place, less blue light is diverted in his direction to overlay the intrinsic blackness of un-illuminated space, and the sky appears, not blue, but a deep, deep purple. And indeed to a latter-day Cortez, observing the sky from a space shuttle right outside the atmosphere, or from the surface of the moon which has no atmosphere to speak of, the sun would appear as pure white, and all the rest of the sky as inky black.