I've done a small check and tried touch-typing with one hand, even though I haven't tried it ever before.
Loaded up Stamina and touch-typed a part of Saul Williams' Coded Language, I've had %9 error in my key-presses. Which, IMHO, is surprisingly low given the fact I can't visually picture a keyboard, all I used was muscle memory from regular 2 hands typing and from gaming, which I assume contributed quite a lot more than my usual 2 hand typing.
Nevertheless, I really can't see the benefits of a blank keyboard. if you aren't ever looking at your keyboard as it is, you won't mind whether it's blank or not. If you're a touch-typist who finds himself in need of learning the various symbols layered on various keys (^"'>~* etc), you'll probably find yourself fully capable of touch-typing them if you do indeed end up using them quite often. So yes, a blank HHK looks infinitely more awesome than a regular keyboard, but other than the coolness factor, what other possible benefits do blank keyboards have?
And no, making the computer impossible to use for anyone who isn't a touch-typist isn't a benefit, not in my opinion at least.