"blank sky" is correct and usable in written English.
It is usually used to describe a sky with no visible clouds, but can also be used to describe a sky with no other visible features, such as stars, birds, etc. Example sentence: As I looked out the window, all I could see was an endless, blank sky.
Stations, bridges and tunnels enter their language and the scribble of brown twigs on a blank sky.
"Daybreak" is a dimly lit landscape under a blank sky, white at the horizon and pale blue above.
Early photographers, many trained as painters, made up for technical limitations by altering their prints — adding color, combining images, filling a blank sky with fluffy clouds.
On the Barabinsk Steppe, both kinds sometimes wheeled in great numbers that vivified the blank sky above the wide-open horizon.
Still, this playful, patriotic, martial costume enlivens the brick walls and blank sky that recede into the background, and calls attention to our subject's resourcefulness — and his individuality.
His footage sweeps round four screens, one on each wall: the wooden barracks, the bleached blank sky, the careening horizon, the snow, telegraph wires and trees.
J.F. finds himself worked into such a rage he's got his goddamn cell phone out, wielding it like a sword or some kind of laser like Han Solo did in Star Wars (you want fantasy, there's some fantasy, motherfuckers, he screams at the blank white sky above him, George Lucas makes all you FC2 Dalkey Archive Borges sluts look like Fight Club!).
John Kolodij created the perfect soundtrack to crumbling scenery and blank skies.
We both look out the window, at the frozen white sky, blank with possibility.
Clouds in a blank pewter sky.
The panels were painted an anodyne blue, like the colour of the blank midday sky.
