The sky as seen from the Earth appears blue because of the scattering of sunlight by the Earth’s atmosphere. When sunlight reaches the Earth’s atmosphere, it hits suspended molecules of air and dust and gets scattered in all directions. Sunlight consists of light waves of varying wavelengths each of which is seen as a different colour. The size of the scattering molecules decides which colour is scattered most. Gas molecules in the atmosphere scatter blue colour the most. So during the day the sky appears blue because more blue light is deflected towards the earth. However, on the Moon, since there is no atmosphere, scattering of sunlight is negligible. So the sky appears black from the Moon.
