It's down to diffuse sky radiation
Rayleigh scattering... its when light passes through a media like the earth atmosphere and by refraction or maybe absorption by the molecules, or maybe both, the light is scattered diffusely(in all directions). The scattered light is blue because when the photons are absorbed by the molecules, just the "blue ones" remain... that happens because the eletronic configuration of the molecule does only allow the molecule to emitt "blue photons".