It has to do with the amount of photons that enter earth's atmosphere and at the particular angle that they enter. In turn causing a dispersion, of various light particles, that give the sky it's blue hue. This is also true of the blu color seen in the bird, the bluejay.
Bluejays, are really not blue in color, but are actually white. Because of the same principle based on the sky being blue. That is also why a bluejay appears blue to the human eye. I've used the bluejay here, as an illustrative comparison.