Editing layer masks with brush

[Besieged Parent]

I regularly run into problems when I need to edit a mask on an adjustment layer. I can delete the area affected by the mask with no problem, but can't seem to ADD bits back to the mask via a brush. My understanding is that I should be able to select the mask in the layers menu and use the black brush to draw additional masked parts.

What am I doing wrong? Example below. (I'm trying to cover some white bits here with more of my aqua mask but the black brush deletes mask rather than adding to it. Note that I click Control + Z to undo the impact of the black brush.)

[MEB]

Hi @Besieged Parent,
Welcome to Affinity Forums
You have the Blend Mode set to Darken (context toolbar). Set it to Normal and it should work as you'd expect.