A basin of clear water (very clear) is irradiated with a flashlight (or laser lamp), and there is basically no obvious light or bright light in the water;
A pot of slightly dirty water is illuminated with a flashlight (or laser lamp), and you can see that the water is "illuminated".
There is almost a vacuum in the universe, and light passes directly without scattering, so it is black; When it hits the atmosphere, it will disperse and the atmosphere will light up.
As for the bright blue atmosphere, it's like pouring blue ink into water. The ozone in the sky is blue.