This blue hour shot was taken in the form of several exposures that were combined to form one image. The exposures were taken at the same f-stop but with different shutter speeds ranging from 1/15th of a second to 15 seconds. Needless to say a tripod is a must for a shot like this because you do not want blur from camera movement and you want all the photos to have exactly the same framing of the image. Photomatix software was used to combine the 5 exposures into one photograph.
HDR (High Dynamic Range) increases the dynamic range of what we can see in a photograph. Our cameras are nowhere near as sensitive to light as the human eye. While our eyes would see all the detail in this scene, a single photographic exposure would have some of the buildings too dark and the capitol dome so white in areas that no detail could be seen. HDR mixes things together from various exposures and creates an image where we can see more dynamic range than can be seen in the normal photograph.