It is well known that blazed optical diffraction gratings can significantly increase the diffraction efficiency of plane waves for a selected angle of incidence. We show that by combining blazing with a phononic band gap, diffraction efficiency approaching 100% can be achieved for acoustic waves. We obtain experimentally 98% diffraction efficiency with a two-dimensional phononic crystal of rotated steel rods of square cross-section immersed in water. This result opens the way toward the design of efficient phononic crystal gratings.