The light from the Sun has to travel a long distance of the earth's atmosphere before reaching us. As light travels through the atmosphere, it gets scattered in different directions by the air molecules present in it's path.
The blue (or violet) light due to its short wavelength is scattered more as compared to the red light of longer wavelength.
Thus, the light reaching our eye directly from Sun is rich in red colour, while the light reaching our eye from all other directions is the scattered blue light.
Therefore, the sky in the direction, other than the direction of Sun, is seen blue.