Or, right click and simply disable them, because you might change your mind and decide to use them later.
However, if you really must put a black solid underneath, just use a slug, which is always available to you at any time by clicking on the little icon below the jog wheel on the right side of the FCP viewer that looks like a film frame with an "A" on it.
Thanks, a slug should make things a little faster. I'd been using the blade tool to separate the video underneath and set it's opacity to 0 but if I want to adjust the length of the picture there's many more steps than if a background automatically filled the space behind the pic.
Does the feature I described not exist in final cut?