Fun little game! Not inherently obvious what I should do, but easy enough to figure out on you own. Also really liked the art style. Really enjoyed that I made the music by placing the blocks. Even though I sucked at making a coherent song :p I would suggest adding some form of goal to make the player driven to play the game more. Good job!
Well if you could make some sort of goal (a #/per second goal or someting) and maybe make different fields and put some restrains on them (like you can only use 3 of a type of block) then you'll have a problem to overcome and it'll become a kind of puzzle game. That could be fun, it'd also help with the retention of the player I would feel :D