2 Square brushes I made while on vacation. I use these for blocking in shapes or for using it as a hard eraser when on alpha lock mode.

The opacity brush changes in opacity with pressure. You can use the lighter pressure for blocking in and a harder pressure for “Validating” the shapes or simply for shading.

The harder brush has a slight Color mixing feature and more of a painterly behaviour. I use this for painting and also blocking in harsher shapes.
You can use either as eraser too if you put it on Alpha Color mode.
Here’s examples of the brushes:

Apple’s square brushes