[yawpitch]

The sky isn’t blue. It’s transparent. That’s why you can see stars that aren’t blue at night. When struck by sunlight at the right angles it appears blue, but saying it is blue is like saying the ocean is green when a bucket of it clearly isn’t.

[jxdxbx]

If something appears blue, it is blue. That’s all color is.

Also, if you took a sufficiently large quantity of air and put it into empty space and shined very bright white through it, it would experience rayleigh scattering—-meaning that air, when you have enough of it and shine a bright enough light through it, is blue.

[yawpitch]

By that logic the sky is near-Planck length ultraviolet, because if I put a sufficiently large quantity of it immediately beside a supernova it goes way past blue.

Color is a property something has under certain conditions, it is not a property of what that something is under all conditions.

[mncharity]

Perhaps "transparent with a blue tint"?

[yawpitch]

I’ll allow “transparent with an occasional blue tint under the current conditions of its nearest star”.