So I'm going to export this project in which I want everything that I color black to be transparent. The composition BG setting is default (i.e. black) and I wanna export it in RGB + Alpha format. Now here's the problem. I have a few black rectangles in my compositions, but they won't be transparent when placed in Final Cut Pro. Meanwhile, the black BG is exported correctly (transparent). How do I fix this? Or do I need to use masks instead?
You probably need to export in layers, particularly the black rectangles as there own export with an alpha channel. I don't work with FCP, but in Avid/Premiere you would import that with an alpha channel and then use that as a matte or stencil in in your final project. For instance, in Avid, if I wanted those rectangles to have different video in them, then my background, i would step into the rectangle matte layer and swap out the fill, or I could use the matte effect. There is probably a similar procedure in FCP
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.
In the future, use Toggle Transparency Grid button (the little checkerboard looking thing) underneath your composition to see what your which parts of your composition are transparent if exported with an alpha channel.