Picking your spot
Blue hour generally starts about 30-45 minutes after sunset. This will vary depending on your location on Earth, and the time of year. If you are a morning person rather than a night person, there is of course another blue hour which starts 30-45 minutes (or more) before sunrise. You probably want to plan to get to your spot just after sunset, to give yourself enough time and light to set up and try some test shots. I have found the best subject for blue hour photos is a well lit building or other structure such as the steamship above. You can also do scenics, but a well lit structure tends to come out better.
I use a Nikon D90, and these settings tend to work for me. Your camera may be different, so these settings should be used as starting points. First of all, you’ll be on a tripod, so turn the AUTO-ISO off, and set ISO to the lowest available to your camera. This will likely be 100-200. Nikon cameras have a low ISO setting called Lo 1 Lo 0.7. You can try these settings if you like, but they are digital hacks, and I find I get great results with ISO 200. If your camera has a mirror lock-up feature, turn it on to ensure the sharpest shots possible.
Now if you are shooting RAW format (which you should be) you don’t need to worry about white balance. Just set it to ‘AUTO’ fire away, and you can easily adjust colour temp in post with your RAW editor. If you are shooting JPG, then you will need to know something about the type of light in your scene. There may be many types of lights which make it difficult to select a ‘correct’ white balance setting in camera. My advice: Try a few different setting for some test shots, and see what looks best. The most common lighting will probably be sodium arc, common in most street lights. Sodium lights will leave a pronounced red cast to your picture which will need to be corrected. Again, if you shoot RAW this will be trivial in post. If shooting JPG you will just need to experiment in the field and try to pick a white balance setting that looks good.
Taking a shot
So you’ve mounted your camera on a tripod, framed your shot, and you’re ready to go. At the start of blue hour, there is still a considerable amount of light. Now don’t be afraid, put your camera on full manual (‘M’ on the dial on Nikon cameras) and set your aperture to about f13 or so. Set the shutter to 4-6 seconds. Now, you should always be using either a remote shutter release or the timer setting. The simple act of tripping the shutter is enough to cause camera shake, and a blurry picture. So: Frame, focus, fire, and review. Too bright? Cut the shutter time or choose a smaller aperture (ie: higher f-number). Too dark? Increase the shutter time or use a wider aperture (ie: lower f-number). Always review your shot and make corrections.
Now as the light gets darker, you will want to ramp up your shutter times and open up your aperture. I like to keep the aperture in the f/11 area for best depth of field, and also, you get nice starbursts from lights in frame with a smaller aperture. Keep taking shots. When you get to 30 seconds at f/11 you will need to start opening your aperture. I keep shooting until I am at 30 seconds at f/3.5. By this time, most of the blue in the sky is gone anyway. You can ‘prolong’ blue hour by shooting on nights with close to a full moon. The moon will light up the sky and reveal blue pretty much all night. I’ve been able to pull blue out of the sky very late at night using a 30s+ shutter and my largest available aperture.
Your AF system may have difficulties focusing at night. A well lit building will probably work fine but a scenic may cause problems. You may have to focus manually, or if your lens has a distance meter, set it to infinity, and everything farther than the lens’ hyperfocal distance will be in good focus.
Doing some post
Now you should have several pictures on your camera, and you should be able to see pronounced blue even on the little LCD straight out of camera. Basically, first you want to adjust white balance/colour temperature. Use your RAW editors slider and see what looks good. Basically, you want to remove any colour casts created by ambient lighting sources. Again, the most common will be red. Move the slider until the red cast is gone, and points of light appear ‘white’. Play with saturation and contrast if you like, and also, raising the gamma a bit may help introduce a better dynamic range to your shot. Last step, run an unsharp mask and you should be good!
Hand-holding blue hour
If you don’t have a tripod or don’t want to use one you can also try to hand-hold a blue hour shot. While you will not get as sharp an image as if you were on a tripod, you can capture other things such as people, who will appear ghosted if at all on a long (2-3 seconds +) exposure, and moving vehicles which will likely show as a car-shaped blur rather than disappear completely leaving just the streaks from their lights as in a longer exposure.
To do this you will need as fast a lens as possible, probably at least an f/1.8. You will also have to crank up your ISO settings which will introduce some noise. The amount of noise is completely dependent on your camera’s high ISO ability. Full-frame camera’s such as the Nikon D700 always do better in this regard. You will also have to deal with relatively long shutter speeds, such as 1/30 of a second or longer. To ensure the sharpest shot possible I recommend placing your back against a wall or other solid surface, bracing the camera stiffly against your face, and slowly exhaling while pressing the shutter. Here’s a blur hour hand-held shot I captured in Edmonton: