Fang Tieyun wasn't beguiled by Bu Tiange, he simply wanted to get back at someone.
He had previously agreed to cooperate with Bu Tiange, but he never actually agreed to join the Black Mountain Demon.
Fang Tieyun was just dissatisfied with the unfairness of Refining Blade, not seeking to gain anything else.
What could he gain by joining the Black Mountain Demon? Nothing but infamy.
Fang Tieyun enjoyed crafting weapons and preferred to see the weapons he forged used by the heroes of the Martial World, not as tools for consuming humans in the hands of Demons.
So, from the beginning, Fang Tieyun never thought about joining the Black Mountain Demon, it was just Bu Tiange being 'kind' wanting to secure a future for him, without applying too much pressure.
Fang Tieyun taking this step meant he had almost gotten his retribution.
After this Weapon Casting Conference, Refining Blade was doomed to become the laughing stock of the Southern Mountain.
