I may be asking the question in an unclear way, because I come from an audio and graphics background. Apologies in advance. Hope this is makes sense...
My Final Cut Pro X project is 960x720, and I'm putting a clip on top of it that's larger, but scaled down using "Spacial conform" so it fits vertically, and there are black stripes to the left and right of the clip.
The clip is a Powerpoint and it's got a white background, so I'm using "Stencil Luma" blend to put a nice animated video behind the text. The effect is nice, but the black bars to the right and left of the clip prevent the animation from showing underneath those black bars.
I simply want those black bars to go away, or become white, so that the background video won't be clipped while the front video is on the timeline.