A clear blue sky — an expanse of sky with few or no clouds, often implying fine weather.
遠足の日は青空が広がって気持ちがよかった。
On the day of the excursion, a blue sky spread out and it felt pleasant.
青空の下で昼ごはんを食べた。
We ate lunch under a blue sky.
写真に映った青空がとてもきれいだ。
The blue sky captured in the photo is very beautiful.