I won't ask why the sky is blue, but I do want to know what people mean when they talk about blue-sky ideas or blue-skying something.
There seem to be several ideas attached to the simple phrase blue sky. Let's start with the most direct one. Lately, there have been blue-sky projects aimed at reducing greenhouse gas emissions and generally cleaning up the environment. It is easy to make the connection between a healthy environment, clean air and blue skies.
Blue-sky also describes laws regulating investments. About a century ago, the state of Kansas enacted the first blue-sky laws. They were designed to protect investors from fraudulent securities with "no more basis than so many feet of 'blue sky.'"1 A blue-sky stock is definitely the opposite of a blue-chip stock, which is the most solid kind!
In business and management jargon, the adjective blue-sky means anything from worthless, impractical and unrealistic to idealistic, innovative and ambitious. Blue-sky is also a verb meaning either to have unrealistic, impractical ideas or to think creatively. In the second sense, blue-skying it can be a synonym for thinking outside the box.
Some writers even use blue-skying it to mean thinking out loud or brainstorming. Of course, the idea of brainstorming is to throw out ideas as they come to us, in the hope of coming up with something creative and innovative. At the same time, many of the ideas produced by brainstorming will be impractical, so there's a connection here with both of the previously cited meanings.
I hope this clears the clouds from your blue skies.
1 United States Supreme Court decision Hall v. Geiger-Jones Co., 242 U.S. 539, 37 S. Ct. 217, 61 L. Ed. 480