Really cool twist on the classic game! It took me more time than I'd like to admit to wrap my head around the empty blocks. For some reason, I kept thinking that there could be mines under the numbers. I think I was thrown off by the fact that blank blocks can appear at the borders of the empty space, which can't happen in the regular game.
Minor gripes, I'd like to be able to use WASD, arrows + mouse is a bit awkward. Also, middle-clicking on a number that's next to that many flags should reveal the surrounding squares. Not taking points off for these but it's a bit tedious to click all of the blocks surrounding a 1.