I honestly don't know for sure why the sky is blue, but I will take my best guess.
I think the sky appears to be blue because the atmosphere absorbs all of the other colors, but reflects the blue. When light shines, it shine millions and millions of different colors, but in the atmosphere only blue appears. This is because the air absorbs the other colors that the light shines, except for blue.
This is the same reason for the sky getting dark when clouds block the sun. When the clouds block the sun, the light rays can no longer bet through, causing there to be no more colors to absorb or reflect.
Another theory is that outer space absorbs all of the colors except for blue --- which it reflects.
I think the sky appears to be blue because the atmosphere absorbs all of the other colors, but reflects the blue. When light shines, it shine millions and millions of different colors, but in the atmosphere only blue appears. This is because the air absorbs the other colors that the light shines, except for blue.
This is the same reason for the sky getting dark when clouds block the sun. When the clouds block the sun, the light rays can no longer bet through, causing there to be no more colors to absorb or reflect.
Another theory is that outer space absorbs all of the colors except for blue --- which it reflects.