Catalyzes the two-electron reduction of biliverdin IX-alpha at the C15 methine bridge. 15,16-dihydrobiliverdin + oxidized 2[4Fe-4S]-[ferredoxin] = biliverdin IXalpha + 2 H(+) + reduced 2[4Fe-4S]-[ferredoxin] Belongs to the HY2 family.