Wow, what a great puzzle game! I really enjoyed how polished and well put together it was. My only complaint was that the music didn't loop, so I got stuck in silence a few times.
I had to come look for spoilers on how to beat the last level. I eventually managed it, but I'm not sure how. It might be a good idea to show an indicator for the path the block takes back for the undo. That would remove some of the trial and error for the puzzles where the number of blocks you push before reaching the exit matters.