I'm a night photography fanatic. Night photography is beautiful and contains a certain mood that other types of photography simply don't contain. Because I love night photography, I spend a lot of time looking on Flickr for night photos. Between viewing night photos online and teaching photographers in my in-person workshops, I see a lot of mistakes being repeated.
There is one misstep that nearly all night photographers make. It seems that this mistake is so common that it is done incorrectly more often than it is done right. The mistake is so disastrous that it makes an otherwise beautiful scene dull and uninspiring, but most photographers don't even know what they did wrong.
What's the mistake? Underexposure. Photographers are used to exposing an image to match what the eye sees, so when they take pictures at night, their photos are just as dark as what they see with their eyes.
Viewers of night photography will understand that the photo is taken at night by the quality of the light. It is unmistakable. There is no need to make large portions of the photo pure black just for a viewer to know that it is night time. One of the most obvious signs that a photo is a night shot is the light coming from street lights or other lamps.
Your night photos will have much more impact and drama if you expose the image properly. If you don't believe me, just go search Flickr for night photography, sit back, and look at how much detail in the scene is wasted simply because the photographer underexposed the image.
One reason why night photos are often underexposed is that DSLRs will only allow up to a 30 second shutter speed without a remote release. Buy a cheap (usually $15) remote release for your DSLR and then you will be able to shoot long shutter speeds which will properly expose the image.