Originally posted by stevebrot The "blue" in the "blue hour" is quite simply a shift in spectral composition similar to that of open shadow vs. direct sun. One can use fixed white balance set to daylight (5500°K) as a baseline* and adjust from there modifying virtual spectral response in PP or something similar. Regardless of the approach used, the actual capture data is of a scene that is spectral deficient at the warm end and will behave as such in post.
The later after sunset, the more blue content in the palette. Collecting the exif values of many blue hour shots from the blue hour photo pool at flickr provides a hint of the light levels: exposure time from around 1 sec. to 15 seconds, at ISO100 with the lens aperture at f8.
Today, 20 minutes after official sunset time: 3 seconds exposure at ISO100 / f8.
Today, 14 minutes after official sunset time: 1.3 seconds exposure at ISO100 / f8.
Today, 2 minute before official sunset time: 1/6th seconds exposure at ISO100 / f5.6.