You can create a fade to black (fade-out) by adding a dissolve at the end of the last segment or at the end of a segment followed by a gap.
The following procedure show how to create a fade to black. To create a fade from black, follow the same instructions but work at the head of the video clip instead of the tail.
A dissolve is added using the default dissolve length specified in the Preferences menu. See Transition Preferences.