Only in a dark room.

Dark backgrounds with light coloured text are an enormous strain on the eyes in daylight or normal lit-room ambient lighting. Most dark background websites or blogs are a complete usability disaster.

That said, the Zune and Windows Phone seem to compensate with very large and clear typefaces. This is must and it's part of what makes the UI work as well as it does.

Black text on white or light-coloured background is by far the easiest in the eyes.

Try reading an all-black background site for 10 to 15 minutes and then go walk around your normally lit house for a while. Notice anything disorienting?