Christine at the Vista Woman blog passed on the neatest information earlier this week! The Blue Hour is a set amount of time early in the morning, as show here, and late in the afternoon where, when photographed, the sky is an incredible blue (looked dark when I took the photo!). There is even a website, bluehoursite, about this that will tell you when your blue hour is!
This one was taken later... see how the blue is beginning to fade. These are straight out of the camera. How cool is that???