If I'm editing a mask with a brush, then I would expect the blend modes to work like they do when I'm normally painting a layer. Examples include:
Brush with White Color and Overlay Blend Mode should help fill in white parts of the mask, without revealing/coloring parts that are already black.
Brush with Black Color and Overlay Blend Mode should help fill in black parts of the mask, without hiding/coloring parts that are already white. Works best with a low flow
For clarity, these do what I expect if I just paint with black and white on a normal pixel layer. But none of the blend modes do anything when painting on a mask.