Images called "skyscapes" often do include clouds or land, but these things can also be excluded or kept to a minimum.
There is often nothing to suggest scale in the art, unless a bit of landscape is included or some phenomena such as the depiction of clouds, precipitation, rainbows and aurorae.
During the day, the sky is seen as a deep blue due to the sunlight reflected on the air.
In the 1950s, Eric Sloane painted many cloudless skyscapes during his stay in Taos, New Mexico.
[1] Another example of a cloudless skyscape artwork is the celestial ceiling mural at Grand Central Terminal in New York City.