The Collaborative International Dictionary of English v.0.48:

Blue-skylaw \Blue"-sky"law`\
   A law enacted to provide for the regulation and supervision
   of investment companies in order to protect the public
   against companies that do not intend to do a fair and honest
   business and that offer investments that do not promise a
   fair return; -- so called because the promises made by some
   investment companies are as boundless or alluring as the blue
   sky, or, perhaps, because designed to clear away the clouds
   and fogs from the simple investor's horizon. [Colloq.]
   [Webster 1913 Suppl.]