First off, your best bet is to color correct your entire film to be natural color BEFORE adding Magic bullet. Whether you use color corrector in FCP or Color you need to balance your blacks and whites and get everything looking smooth and nice. Then, one that is done add your Magic Bullet look.
Do you know Color? That would be the way to go because you can easily crush your blacks and raise your white levels until they look good and are balanced correctly. The same thing can be done with the 3-way color corrector in FCP, but turn on your waveforms and check your black levels and white levels.
Best of luck.