I have two images stacked on top of one another and applied a layer mask to the top layer so I can mask out a section to show the bottom layer. I've done this a hundred times but for some reason now when I paint black on my mask it's not revealing all of the layer underneath. It's transparent. My blending mode is on normal, flow and opacity are 100%. I'm at a loss. Any suggestions?
Sounds like you're doing everything right. Did you accidentally select inverse? Are you painting/filling with white accidentally? Is your airbrush set to normal? Start a new document and try again from scratch and see if the problem occurs again.