The Xbox One is not just black. It is "liquid black". There's an element of marketing fluff behind this, of course, and the console's design team are not short on marketing fluff, but behind all that, there's some interesting reasons for the decision.
Here's an excerpt from a design piece on Microsoft's new "Xbox Wire" site that explains why not just the console, but the controller as well, are so dark.
The console and Kinect sensor are liquid black so they melt into the background when being used, allowing the content on your TV to dominate the living room. The user interface is overlaid on the same shade of deep black so that the content tiles on the dashboard are more vivid and easier to navigate and interact with.
Even the wireless controller uses liquid black to enhance visibility. The traditional A, B, X and Y buttons were re-designed so that the coloured letters now float on a sea of rich, liquid black, a “triple-shot of injected resin,” clearly defining their placement on the controller.
Like I said, there's fluff in there, but also some genuine practicality. If you're not put off by design talk like that, there's more at the full report, which strikes a nice balance between interesting approach and complete bullshit.