Newbie Question: Simple Layer Mask I am trying to paint in Black or White and I only get gray

Hi Folks,

I am embarassed to ask this question but I have been searching for an hour and just can't find an answer.
Details:
PS 2021, MacOS
Two Layers
Reveal All Layer Mask on Top Layer 
Brush set to: Hardness 100, Opacity 100, Flow 100

I start painting black and I get a 50% grey.

Screenshot attached

If I change the Layer Mask to Hide All and paint white again I get a 50% grey and can't paint pure white.

It has to be something incredibly stupid that I am missing but I can't figure it out.

Thanks