Color Stabilizer on white keyed footage

Hello guys,
hope you can help me, I have got several (8) stop motion footage moving in a 3d space in the comp.

They're keyed out (so they have alpha channel) clothes shoot and they're all mainly white, basically I'm not sure where to pick the black point...
I just need the black one since I'm adjusting just the brightness
So when I apply the effect (is it possible to apply it to an adjustment layer?) and I render it out nothing happens... Do you have any hints to make it work?

Thanks alot.