To understand the blue-tinted mountain phenomena, it's crucial to first understand how light works and where that light comes from: the Sun. Sun rays appear white because they include all the colors in the rainbow, just mixed up, as HowStuffWorks explains. Even though all the light blends into a blinding white light in a sunbeam, each one of those colors travels on its own wavelength, some of which are longer than others.
Once that light hits the Earth's atmosphere, though, and once it interacts with air molecules, those light wavelengths from the sun begin to bounce around — the shorter the wavelength, the more movement it has. This is called Rayleigh scattering. Among the smallest of all light wavelengths are blue and violet, but sunlight has far less violet in it than it does blue. This phenomenon is also partially why the sky itself appears that shade of color, and it also explains why mountains can sometimes look blue at a distance, but it's not the whole story.
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