Location: Tianshui City
Petitioner: Li Sheng the Blacksmith
Reward: Yuchang Sword, White Tiger Sword Advanced Blueprint
In order to trigger this side quest you need to gain access to Tianshui City in the first place. This requires completing a significant number of main story quests, all the way up to Birthday Banquet in Tianshui. Once you've completed this quest you can cross the western border of Fancheng City and proceed further west to Tianshui.
The petitioner, Li Sheng, is standing at his stall at the entrance to Tianshui. He'll try to sell Yi a blade, only to be interrupted by Ouyang Ju - a relative of Ouyang Xue - who is standing nearby. Ju claims that Sheng is trying to sell garbage weapons, and he's willing to prove it - if you can bring him a piece of Black Gold Ore. (Keep in mind that if you turn this quest down even once it will end altogether, so... don't.)
Black Gold Ore is not terribly rare, and the chances are good that you already have some in your inventory. If not, the closest place you can reliably harvest some is at the Precipice. It's near the Xungu Pass, in the mountains to the northeast of Fancheng City. Bring the ore back to Ouyang Ju to complete the quest.