Refers to a clear, blue sky. Often used in poetic or descriptive contexts, but also common in everyday speech.
今日は青空が広がっている。
The blue sky is spreading out today.
青空の下でピクニックを楽しんだ。
We enjoyed a picnic under the blue sky.
A blue sky stretched out beyond the window.
We had a picnic under the blue sky.
He looked up at the blue sky and took a deep breath.