It is comparatively easier to take portrait shot in bright day light than in dark or low light condition. Taking portrait shot in dark or low light condition is challenging because you have to adjust two lights while taking picture; one is ambient light and the other one is flash light. In daylight shooting, you don't have to worry much about flash light (except fill light) and you don't have to deal much with shadows and colors of different lights. But while shooting in dark, you have to be extra careful with presence of very few or no ambient light, different artificial colors (street lights or other artificial lights in the environment), colors of those lights, shadows etc. Taking portraits at night is difficult but fun as well. As I mentioned earlier, you have to understand how ambient light and flash work together. Ambient light is whatever light present in the environment and flash light is artificial light produced by using light strobe or flash gun. The flash light has a very short duration and thus shutter speed has no role in the overall exposure it gives. You have to remember that aperture controls flash lights and shutter speed controls ambient light; this will make life little bit easier. But having said that, it doesn't mean that aperture can't be used to control ambient light. It's just simple and easy to implement if you remember those theories. Therefore, any shutter speed slower than the maximum shutter speed (also called Flash Sync Speed) can be used with flash. Please keep in mind that different camera model has different "max sync speed" value for shutter; usually 1/200th of sec or 1/250th of sec.

Focal Length : 55mm  Shutter Speed : 1/60th of sec  Aperture : f/4   ISO : 200

You can experiment with different aperture value to adjust flash lights and change shutter speed to change the brightness of the background. The longer the exposure, more ambient light passes through the lens and hence producing brighter background. Sometimes, while working in auto mode (TTL flash), you might not get result you want and it's better to control your camera and flash manually. You also have to be careful about the choice of the lens. Normally, longer lenses are preferred for portrait shot.

Few things to experiment

1. You can try wide open aperture to create nice and soft bokeh in the background which also helps throwing confusing background out of focus. You may have to adjust your light settings because wider aperture allow more light into the sensor.

2. Location is not that much important for portrait as your main subject covers almost entire frame (most of the time).

3. You can also try off-camera flash so that you can play with direction and quality of lights.

4. You can also use colored gel which gives different mood to the photographs depending on which color you are using.

5. If you get lots of shadows, you may want to try bounce flash instead of straight flash.

6. You can also use kicker flash (normally used behind the subject which helps to separate subject from the background).

Focal Length : 55mm  Shutter Speed : 1/5th of sec  Aperture : f/4   ISO : 1600

You can see the difference in Shutter speed and ISO value (among these two photographs) which changed the amount of light coming to the sensor and hence changing overall brightness of the picture. You also have to be careful about choosing ISO value; higher the ISO value, more digital noise (grainy dots) will be present in the image. If you want to learn more about the ISO and the digital noise, you may want to read my previous posts Understanding ISO in Digital Photography and Dealing with Digital Noise.