THE Karate Kid comes to know his mojo in the calamitous "Crossroads," a misbegotten mishmash with Ralph Macchio as a Juilliard guitarist who exorcises a bedeviled old Delta blues man.

In blues legend, aspiring musicians sell their souls at the Crossroads in return for fame. But the alluring premise never really comes clear here under the incoherent direction of Walter ("48 Hrs.") Hill.

And Macchio is completely miscast as well as continually outclassed by costar Joe Seneca, a songwriter/actor who's the film's only grace note. Seneca plays an 80-year-old harmonicat who sold his soul at that mythic Mississippi intersection ages ago. Now the former music great is an unsung patient in a nursing home who persuades Macchio, a lover of his music, to take him south for a showdown with Beelzebub.

Seneca initiates the wet-behind-the-ears guitar plucker into the mysteries of blues and black culture (much as Pat Morita taught the Kid about the Japanese and karate). And the the inevitable racist muck is stirred up by the hacks in charge here.

Fat white bigots and fat black bigots are thick on the landscape as our heroes plod from New York south to Memphis and on to Fulton Hill, Miss. When things couldn't look worse, the dread Jami Gertz, as a slatternly teen-age runaway, joins them on the road, and Macchio, having just discovered the guitar pick, falls in love.

The manufactured romance allows Macchio to feel the pain of parting when the little tart -- a thieving, gun-crazed little wretch -- hitches on to L.A. This lets Seneca utter the proverbial, "Blues ain't nothing but a good man feeling bad."

And as we all know, you can't keep a good man down. And so, the spurned Macchio releases all that stored-up pain (he and the girl had been together a couple of days) and, well, he wails. It happens. He's a blues man.

Gertz gives a clumsy performance and Macchio has got all the soul of a Spaghettio. Suitable as Seneca is, his character, a foul- mouthed old crackpot, is too sour to cozy up to.

Just when you think you've got them all sorted out, Hill introduces a heavy metal guitarist with brimstone on his breath. To save his mentor, Macchio challenges the devil to a musical duel, and the movie becomes a visual version of "The Devil Went Down to Georgia."

The riffs are smoking. The devil dances around little, limp Macchio, sneering and chording. The metal grinds. The sweat flies. Macchio plays Mozart, proving that rock is the instrument of the devil. But the scene evaporates idiotically. Turns out there was nothing to fret about. And even less reason to go.