Catalyzes the two-electron reduction of biliverdin IX-alpha to the tetrapyrrole chromophore phytochromobilin (PPhiB). (3Z)-phytochromobilin + 2 oxidized [2Fe-2S]-[ferredoxin] = biliverdin IXalpha + 2 H(+) + 2 reduced [2Fe-2S]-[ferredoxin] Elongated hypocotyl phenotype due to defect in the tetrapyrrole chromophore phytochromobilin biosynthesis. Belongs to the HY2 family.