I can't test my brush when editing it and switching to some modes, including the "greater" blending mode.
That's because those blending modes need something to blend with, so you'd need to create a background that alles the blending mode to act.
yes, this is because the scratchpad is not transparent but filled with white pixels. This will require a rethinking of how the scratchpad works.
It's actually possible to erase the white pixels in the scratchpad.