According to YG, Grade 4 at Cedarwood. But that's Iron/Cobalt now.

Been leveling gsm off Dated Darksilver Rings (well, dated electrum now) which is two silver nuggets and a pinch of Brimstone.

Maybe Darksteel will be the same as the Steel recipe but with an added hard-to-find ingredient?

Two Iron Ore, one Bomb Ash and the mystery item?