It was ok
What I didn't like was that I didn't even hit a white cube it was game over. That's all I noticed, it's a good idea. If you perfected it, that would be great and you'd get a 10 from me :)
it's a start
Just throwing in some music can greatly improve how your game is received. Also, The black cubes appear on screen as soon as you touch the previous one, which is kind of lame. I would suggest placing the X axis of possible spawn points for the black cubes up higher. If you do that, there will be some small wait before you see one again, and it looks nicer because the cubes fall into the frame as opposed to just showing up.
I do like the game though, it's very similar to a project I myself worked on, Zombie Marv. I think you should check it out, and see if it gives you any ideas. I would be more than happy to help you, if it does.
You have some points to work on before you put a game up
#1 ACCURACY: Even if you don't touch the white dot you get a game over!
#2 IDEA: You need a better concept than moving a 3 way cross hair across a black dot
#3: CREATIVITY: This has happened too many times BE ORIGINAL!
Just kidding, but seriously though, This game could have a better presentation and maybe more challenge and depth, like having the black blocks appear from different sides of the screens, and maybe adding items to help evade the white blocks.
no sound, not much concept, there just isnt much in this to make it fun and exciting.