I don’t know if masks work different in BorisFX as with other programs where black conceals and white reveals. When I apply an effect (lens flare) I add a mask so I can paint out the light streaks that I don’t want instead of disabling boxes in the lens flare editor. I invert the mask so it is white and it should reveal the effect so I could paint out with black brush but this is not what happens The white mask acts the same as the black one. I think it is a software glitch but I have to click only once with the brush on the white masks and it reveals the effect.
It would be also much more intuitive to put a black and white brush option in the upper menu instead of add and substract as many of us do all the time paint in black and white to refine masks.