One must assume that, beyond the dance floor, BLAKE BECKHAM'S life is not a flawless expression of perfect intentions. Surely she sometimes hesitates, missteps, maybe stumbles or stubs a toe. But when she dances, Beckham moves with the sublime certainty and manifest rightness of a religious revelation. All moves as it must, her body seems to say. And like the orbits and rotations of other heavenly bodies, her marvelous geometries reveal intimate infinities in the interplay of gravity, momentum and light.