Ryan Dubey suggests that getting some late-night (or early morning) trail runs will help your overall running speed.
As you get more experience in the dark, it gets easier and more natural. Any bad habit is amplified. If you are in the habit of not picking up your feet or being sloppy with foot placement, you will be cured of this quickly. As you learn to do these things better, your daylight runs will get faster. You start to make quicker decisions as to foot placement. With the better visibility, you can look ahead and pick your line sooner. This lets you run faster as you already know where your feet are going to land.