The Blue Hour is really the Blue 15 minutes. It is a very magical time of day for photography and it actually takes place twice in a 24 hour period. Depending upon ones latitude and whether the camera is pointing towards or away from the direction of sunset in the evening or sunrise in the morning, there is a magical 15 minute period, where in with one exposure you can get an incredible balance between the warm artificial light and the intense blue of the sky. In Central & Southern Europe, where we do most of our blue hour photography, that 15 minutes is between 45-30 minutes before sunrise or 30-45 minutes past sunset. You will need to use a tripod as exposures can be as long as 30 seconds.
