Light From the Sun Contains all the Colors. Blue Light has the Shortest Wavelength of the Visible Light.
It is Scattered more in the Earth’s Air. It is called Rayleigh Scattering.

Scattering of visible light photons by Nitrogen and Oxygen molecules in the atmosphere. Rayleigh scattering — the cross section for scattering goes like wavelength^-4. The blue light is scattered sunlight.

Scattered light from the stars and moon also makes the sky faintly blue at night, but the surface brightness is below the threshold of human color sensitivity.