Cutting down bushes leaves black square?

You need to set the screen's Undercombo. Select the tile you want to use as the undercombo, and go on Data-> Undercomdo and set as undercombo.

The Undercombo also applies to other things, such as armos statues and push blocks

There is now a Slash>Next combo that changes to a next combo in the combo table so you don't have to set the undercombos. It's great for having multi undercombos.