Blue hour is the shorter window just before sunrise or just after sunset when the sun is below the horizon but still indirectly lighting the sky a cool, even blue.
Blue hour is the short stretch before sunrise or after sunset when the sky still glows but the sun is gone, and it is the window that makes cityscapes and architecture work. During it, the sky's brightness roughly matches the artificial lights coming on in buildings, so one exposure can hold a deep blue sky and lit windows without either clipping or going black. That balance lasts about twenty to thirty minutes and then it is over.
Arrive late for a skyline shot and the sky reads as flat black behind the buildings; arrive early and the sky is still too bright for the lights to register at all. The colour also moves fast inside the window, from a pale cyan just after sunset to a deep navy near the end, so the same composition shot ten minutes apart gives two clearly different moods to choose between.
A photographer setting up to shoot a lit bridge arrives forty minutes before sunset to lock the composition on a tripod. Right at sunset the sky is bright and the bridge lights barely show, needing f/8 at 1/15s, ISO 100. Twenty minutes later, mid blue hour, the sky has dropped to a rich blue and the bridge lights read clearly, with the exposure down to about 4 seconds at the same aperture.
By forty minutes after sunset the sky is nearly black, the contrast between the lit bridge and the empty sky looks harsh, and the frame has lost the colour that made it worth doing. Every keeper from the session came from a fifteen-minute stretch in the middle, which is why the tripod goes down early. Blending a separate exposure for the sky was unnecessary here, because a single frame's dynamic range covered the whole scene once the sky and lights were close in brightness.
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