black box

@Peter - The Low Ceiling effect is a feature of the game. You can see it in action in level #2 - "Run of the Mill." I wrote about it as well in the Introduction to Mekorama document found here. What @mik did extra here was use the Inset Stair trick in combination with it. The Inset Stair on its own essentially prevents B from passing to the block beyond it - and that is why you can't simply tap a spot beyond the black box in order to send B to it.

The Inset Stair works unless B is moving under the influence of his Autopilot routine. One way to implement the Autopilot is to use the Low Ceiling effect. In this case, the inset stair is the first block under the ceiling in each doorway and is thus the only block that can be tapped. But since the inset stair is under a low ceiling, tapping it will create an autopilot calculation for B and then send him on "autopilot" on that course, i.e. to the exit beyond.