The BlackBerry 10 devices utilizing OLED screens have a native dark theme. BlackBerry redesigned a few of the native applications to make use of the dark theme.
One of those redesigned apps was the native clock. The dark clock looks pretty sleek. Besides the color inversion, the only other change is the timer has been redesigned.
Have a look at the only ‘black clock’ you’ll find on BlackBerry 10.1 in the video below: