Can we use 青空 and 青い空 interchangeably?
Is there any difference between the two? Which is the most natural?
In this specific case, yes, 青空 and 青い空 both mean "blue sky". When used on their own, there is no surprising connotation and thus they are interchangeable. But 青空～ in a compound noun can mean "open-air" or "outdoor" (e.g., 青空教室 is not the same as 青い空教室).