Craig Hockenberry of The Iconfactory provides some good background on how OLED displays work and how that may inform Apple’s design choices on the Apple Watch:
One of my first impressions of the Apple Watch user interface was that it used a lot of black. This makes the face of the device feel more expansive because you can’t see the edges. But more importantly, those black pixels are saving power and extending the life of the display. It’s rare that engineering and design goals can align so perfectly.
Because OLEDs only use energy when pixels are lit up, devices with predominantly black interfaces can last longer.