When I cast darkfire on my weapon and examine it it says temporary on hit flame weapon level "X", it should say darkfire instead of flame weapon. This would fix the problem where flame weapon and darkfire does not stack. (Unless they are meant to be unstackable)

Only one OnHitCastSpell property is effective at a time, anyways. Darkfire OR Flame Weapon is meant to stack with the Metal Domain Power, and they do.

It's odd that Darkfire isn't using its own effect, though.

That's because Darkfire and Flame Weapon were supposed to do 1d2 base fire damage. Due to how the system works, that wasn't possible so now Darkfire uses Flame Weapon's OnHit property for 1d4 base fire damage instead.