In my eyes, the best low light photography isn’t done in darkness, it’s done just before, just after the sun has set, but before the sky goes it’s inky black. It’s for that brief period of time when the sky goes a lovely cobalt blue colour, and you can really make a feature of the sky as part of your composition. Sure, I’ve taken some very nice photos with a black sky, but I think they work well because the black helps emphasise the other colours in the scene, as opposed to adding anything to the scene in itself.

http://en.wikipedia.org/wiki/Twilight

This photo I took of the Victor at Elvington is a good example of the sky adding something extra to the photo:

This one is another where the colour of the sky becomes a compositional element:

Of course, sometimes, the black of night is just what is needed to add contrast to a scene. The garish colours of Blackpool need that black in the sky, I think.