Catalyzes the two-electron reduction of the C2 and C3(1) diene system of 15,16-dihydrobiliverdin. (3Z)-phycoerythrobilin + oxidized 2[4Fe-4S]-[ferredoxin] = 15,16-dihydrobiliverdin + 2 H(+) + reduced 2[4Fe-4S]-[ferredoxin] Belongs to the HY2 family.