Back in March of 2004, I posted to the Stadsbygd mailing list a story from Adresseavisen called Mysteriet med den blå døren (The mystery of the blue door).
The reporter traveled the roads of Stadsbygd and found external doors painted blue on some 30 farms, mostly in the Rein, Fenstad, Brødreskift and Askjem groups of farms. The reporter interviewed many people but did not come up with anything conclusive about the tradition. One theory given was that it may have been started by fishermen from Stadsbygd who made good money during the 1800s, and when they came home they wanted to spend their money on something that showed that they had been successful. Blue paint was expensive and therefore a way to show that you had money.
The original link provided for the story has been moved and now may be found here and the pictures may be found here. Another short story with picture may be found here. Thanks to Inger Selven Watts for the original tip, to Arne Langmo for the new links, and to Stig Trygstad for information about the original blue color (see embedded picture above).