There are some fantastic puzzle games on iOS, but the vast majority could be replicated on a different, non-touch device without too many issues. Blek is a rare breed in that it absolutely could not be played without a touchscreen, since your ability to draw on the screen is essentially what the game is all about.
Your singular goal in Blek is to eliminate colored spheres from the screen. You accomplish this by drawing a line on the screen which then endlessly repeats until it either runs off the top or bottom of the screen, or runs into a black sphere. You can draw your line long or short and make it as complicated or simple as you want.
Wherever your line drawing ends is where the next one will start, which means you usually can't just scribble a quick line and let it go, or you'll risk a misguided shot. The first dozen or so levels are very straightforward, but the difficulty quickly ramps up with levels that are downright sinister. You'll go from drawing short lines to ridiculously complex patterns in the hopes of touching all the colored blobs on your screen, and you'll retry later levels dozens of times before getting it right.
The entire experience is very zen-like in that there are no text instructions, prompts, or needless menus. You can instantly recover from a failure by drawing a new line with zero waiting, which makes your inevitable mess-ups sting a little bit less. When you complete a level, a new level appears within seconds, turning a 5-minute time killer into a 45-minute marathon session before you know it.
The best part is that there's really no penalty for failure here. You won't see a counter of the number of times you've fallen short, and the game doesn't antagonize you whatsoever. Losing means you get to try again until you get it right, which is a refreshing twist to what would otherwise be a very evil puzzle game.
Blek has an entire trophy case of awards already, and it definitely deserves all the attention it has been getting. For US$2.99, you can't ask for a more engrossing, yet relaxing experience. The core game features 60 levels which will take you a good while to complete, and the developers note that there are more on the way soon. It's definitely worth the price of admission.