It's a bit more sopisticated than "whack all the black keys." A pianist (at least I would do it this way) would often voice a C7alt chord with the 3rd-7th tritone in the left hand (E and Bb) and the #9 and b13 in the right (D# and Ab), often with the root in the middle; it would look like E and Bb in the left hand and an Ab triad with the Eb on top in the right hand. The "formula" is: the dominant 7th tritone with the b13 triad on top.