Can't create transparency in layer mask

I'm doing something stupid cause I know this is possible.  Trying to blend one photo layer into another layer using a mask.  Have mask set to "normal."  Using a paintbrush set to black.

When I paint, I'm getting red where the top layer should be transparent to the bottom layer.  Get the same red when I try and blend with a marquee tool... or use a gradient to blend. Here's a screenshot...  help!  You can see the red in the upper left corner of the photo where it should be transparent.