The time between sunset and complete darkness is known in photographic circles as "the blue hour".
The time between sunset and complete darkness is known in photographic circles as “the blue hour”, even though it’s not actually an hour long.
The name “blue hour” comes from the fact that the intensity of the blue sky light is at its peak once the sun is no longer illuminating the landscape, and scenes containing (for example) artificially lighted houses are particularly picturesque. The effect is also visible on cloudy days, although the softer, dispersed light caused by the cloud cover reduces the effect slightly immediately after sunset.
By photographing night scenes whilst there is still colour in the sky, the overall effect is much more pleasing to the eye than the same scene featuring a black sky. That’s why you’ll see more experienced landscape photographers out with their tripods at this time of the evening than at night time.
The reciprocity law incurred by using uncorrected longer exposure times also adds slightly to the natural, unedited saturation of the image, giving them a slightly different feel to regular daylight images.
Here are some examples of the effect from my Flickr photo stream, to demonstrate the varying degree of sky saturation according to exposure, weather conditions, and the length of time after sunset when the image was taken.