Black makes a great background color for computer screens. Using a black background brings out foreground colors and allows the eye to see a wider variety of subtle differences. Notice how the page designers for Revo Sunglasses use black. Or look at the use of yellow on black in the Web page for the movie Deep Impact.
Finally, notice how the lack of color stands out at blue moon deSign & brand iDentity. The use of just a bit of color for contrast can also draw your attention to part of a web page, as in this example from a professional photographer: www.cutting.com.