While not technically black (it is basically empty space), you can add a gap clip (option-W).
You can change the duration of a gap by dragging the extremities.
For most purposes this is what you want (the background defaults to black, so "nothing" will be displayed as a black frame, unless you change the background to white or checkerboard in preferences).
In case you actually want to put a black clip in there, use the custom generator.
Message was edited by: Luis Sequeira1