Ahhhh, thank you! I was just about to post here myself with a similar problem -- all my custom brush tip material brushes just stayed black no matter what. It didn't occur to me to convert the layer to an image material layer! By setting it to MONO instead of GRAY I now finally have what I wanted: a custom tip, non-dual-colored, "color blend" disabled brush.
(Because, for some reason, if I had "color blend" checked, my custom brush tip brushes responded to my color choices. The second I turned it off, it switched to black and it stayed black.)
Really appreciate you sharing that. I was convinced it was a bug.