How to get black background "behind" secondary timeline b-roll?

Can someone help me with how I can quickly set all my b-roll (photos and video), above main timeline to have a black background by default? Right now I can see the main talking head interviews overlapping and don't want to fix each frame one by one.2 answersLikeComment

You can either put a black generator underneath them, or prep them in a graphics application so they’re the right aspect ratio for the screen with a black border as needed. 

You can also cut out the video underneath, being careful to also not cut out the audio (I always detach the audio from the video as a general rule).

There's no way to do this automatically in any NLE if the image you're dropping in is smaller than the underlying video.

Thanks all, I got the answer from another forum. The way I fixed it was to add a custom title generator (top left, titles, generator, custom, solid, black) and inserted it below each block/group of b-roll that was in the secondary timeline (which I had grouped with cmd-G). Then adjust the length of the black generator to match the b-roll group. This was pretty fast.

Or build a custom Generator in Motion that has a black background layer.