I think the sky is blue because it's a shift from black through purple to blue,

"I think the sky is blue because it's a shift from black through purple
to blue, and it has to do with where the light is. You know, the
farther we get into darkness, and there's a shifting of color of light
into the blueness, and I think as you go farther and farther away from
the reflected light we have from the sun or the light that's bouncing
off this earth, uh, the darker it gets ... I think if you look at the
color scale, you start at black, move it through purple, move it on
out, it's the shifting of color. We mentioned before about the stars
singing, and that's one of the effects of the shifting of colors."
-- Pat Robertson, The 700 Club