It’s a common misconception that the sky is blue because it reflects the blue of the seas and oceans. In fact, it’s the Earth’s atmosphere, and a process known as 'scattering', that causes our skies to be blue. As white light passes through our atmosphere, tiny air molecules cause it to ‘scatter’. The scattering caused by these tiny air molecules (known as Rayleigh scattering) increases as the wavelength of light decreases. Violet and blue light have the shortest wavelengths and red light has the longest. Therefore, blue light is scattered more than red light and the sky appears blue during the day.

Source: Royal Museums Greenwhich