Adding or generating black video in after effects

I should also tell you that if you want a background color you should not rely on the Comp background color. Almost all codecs will render transparency caused by no background image or solid layer as black. The best procedure is to put a black layer on the bottom of the timeline.

Nothing to do. AE will fill any gap with its default composition color, which incidentally is black already. Other than that you can always add a comp-sized solid in whatever color you fancy.

Mylenium