The composite turbomachine blade of the present invention comprises a preform made of yarns or fibers woven in three dimensions and a binder maintaining the relative disposition between the yarns of the preform. Said preform is made up of warp yarns and of weft yarns, the direction of the warp yarns forming the longitudinal direction of the preform. In characteristic manner, said preform comprises at least a first portion made using a first weave forming the airfoil of the blade, and a second portion made using a second weave forming the root of the blade, and the first and second portions are united by a transition zone in which the first weave is progressively modified to end up with the second weave, thereby obtaining a reduction at least in the thickness of the blade between the second portion and the first portion. The invention is applicable to making a fan blade for a turbojet.