DIED. William Clarence (Billy) Eckstine, 78, singer; in Pittsburgh, Pennsylvania. At the peak of "Mr. B's" postwar popularity, aficionados of cool cloned his fashion trademarks: the jacket hung casually from the shoulders, the rolled shirt collar. But no one could duplicate the honeyed, baritone vibrato that turned Everything I Have Is Yours, Fools Rush In and I Apologize into romantic standards. Eckstine became pop music's first black male sex symbol. Earlier, as guiding spirit of the Billy Eckstine Band, he brought together such patriarchs of bebop as Charlie Parker and Dizzy Gillespie.