It seems that most people say the sky is blue because of the refraction
of the sun's light through the atmosphere.
If that were true, then we would see a rainbow around the sun.
If that were true, then all points near the equator, because of the
angle of incidence of the sun's rays with the atmosphere, would have
black skies, at least near the sun.
If that were true, dawn and twilight would not exist.
And if that were true, again because of the feeble refractive index of
gases, there could never be blue seen near the horizon at mid-day.
The sky is blue because the water fluoresces under incident ionic radiation, provided by the solar wind. This explains why astronauts see a bright, glowing band of blue looking down across the earth's border.
If you have never seen images of an operating nuclear reactor, you should google it. When water molecules are bombarded with radioactive (ionic) particles, they always glow a distinctive light blue - just the color of our sky. It is because we have water in the atmosphere that our planet produces that lovely blue glow in the sky.