This is one article that every serious photographer should read. It deals with using a flash outdoors as fill (or a primary light source) and steps through the process of proper exposure. Do yourself a favor and go read the Balancing Light: Twilight post.

Be sure to pay attention to what David says about exposure. By using Manual exposure and changing the shutter speed you are controlling the look of the background. One key principle of flash photography is that the Aperture controls the exposure for the flash. All the shutter does is open up fully for the flash to fire.

Try this for yourself one evening. Set the camera to Manual exposure. Meter the scene and use an aperture of F4 or F5.6 to start. Turn on your flash and set it to TTL, iTTL for Nikons, ETTL for Canons, or just auto mode. Shoot at the metered shutter speed and also at one or two speeds up or down the scale. Note the difference in the background.

The primary point of this Strobist article is that you have control over the background and can make it light and airy or dark and moody. Users choice. Armed with a little knowledge and an external flash on your DSLR, you have lots of freedom to get the image in your mind recorded on the digital sensor.