Simple idea but well-executed! It's interesting that the "unstable" comes from construction.
The UI was a bit confused at the beginning, but then I understood that a queue of blocks to drop.
The countdown display is raw but acceptable. I don't know if it's feasible to be shorter for those levels which are easy to pass.
Besides, it maybe useful if a button could be provided to exit current level.
The black/white fits the hard mode challenge and it could be a mobile game with polished graphics.