Mask Color

I am working with bracketed photos. I have two images stacked in the layers panel. Dark layer on top. I add a mask to the top layer and select black as the color t use to reveal the bottom layer. This seems to work fine but when I click "X" to switch from black to white the brush dows not bring back what I previously painted.

I hope this makes sense. I know its operator error because I'm very new to Photoshop.

Thanks for your help.