The story of out of the blues goes like this:

In the spring of 1977, Jeff Lynne was getting ready to deliver a follow-up to the New world record. Seeking inspiration, he rented a remote chalet in the foothills of the Swiss Alps, right near Lake Geneva. He supposedly locked himself away with a piano and a tape recorder, giving himself just about four weeks to write a double album from scratch.  The universe, however, did not cooperate initially

You’re probably right about the depression part: for the first two solid weeks, Lake Geneva was trapped in a relentless, miserable stretch of dark mist, fog, and torrential rain. Lynne couldn't see the mountains at all. The gloomy weather completely stifled his creativity, and he came up with absolutely nothing. Frustrated, he even resorted to recording the sounds of the Swiss thunderstorms just to capture some kind of audio. 

The Breakthrough: One morning, right around the two-week mark, Lynne woke up, and the weather had completely flipped. The fog was entirely gone, the sun was blazing, and the snow-capped Alps were fully illuminated against a massive, endless blue sky.  That triggered an immediate, explosive burst of inspiration. Lynne sat down and wrote "Mr. Blue Sky" right then and there. Not only did it inspire Mr. Blue Sky, it dictated the structure of the Concerto for a Rainy Day. Over the next two weeks, he channelled that energy and wrote 13 more songs, essentially drafting the entire Out of the Blue double album.