What phenomenon causes the sky's pervasive blue hue on an average, non-eventful day?

Answer

Rayleigh scattering

The sky's pervasive blue color is primarily the result of Rayleigh scattering, where atmospheric gas molecules efficiently scatter shorter wavelengths of visible light, like blue and violet, in all directions.

What phenomenon causes the sky's pervasive blue hue on an average, non-eventful day?
appearanceskyatmospherechange