OK, I've got it. This new block is called a "Rotator". It has four arrows that appear in sequence on the top. These either rotate after so many seconds, or they change randomly. The point is that B can only move in the direction of the arrows. So if the Rotator block is ahead of B and the arrow also points ahead, he can move that way. But if the arrow points left, he can only move left! And so on. Here, take a look at these mockups I made for each arrow. In the first image B can't move ahead, only right. If he steps on the block he is automatically moved onto the next block to the right.
Then let's add a button that can control the "rotator" direction, that can be pushed by all bots. If we can't control it then all level that use this "rotator" block only can be solved by luck
Great idea! Not luck if the block isn't random, then just timing. B has to wait for the block (arrows) to turn. What if... B can stand on the block? You could watch as he spins round!