While investigating the first Saints camp for Balance of Power, you will slay the Saints Bandit Leader. In his corpse is a note called Notes on the Blacksmith, which gives details about the blacksmith Joften and his ability to forge Golden & Dark equipment. However, it would appear the bandits lost patience with Joften, who became increasingly insane, and ended up killing him. His notes, however, are hidden and would likely reveal his secrets. The only hint given was that as a child, Joften used to play in the area around Half-Moon Mill, so the bandits had intended to investigate there.
Head to Half-Moon Mill and search under the bridge to find a chest containing Joften's Notes as well as other loot. Reading the journal will complete the quest and allow you to craft Golden/Dark equipment once you have the Daedric Smithing perk.
The chest and the note will be there regardless of whether the quest has been started, so if the player did not loot the note that starts the quest and the Saints Bandit Leader's corpse despawned, they can still learn Golden and Dark smithing.