From the documentation:
http://www.escapefromundeath.com/documentation/mechanics/cleric-domains
Metal Domain Spells: shelgarn's persistent blade (1), darkfire (2), keen edge (5) Abilities: 1/day, add temporary fire damage to your weapon for one round/level: +1 Fire damage for non-hammer weapons, and +1d4 Fire damage for hammers. This should stack with darkfire. [emphasis added]
I've tested this out with my dwarf cleric. It doesn't.
I admit this isn't a huge deal: the buff only lasts for a handful of rounds. So maybe you should just change the documentation to what the code does. Either way. It just should be consistent.
This probably got changed recently when they stopped regular elemental damage on weapons from stacking with darkfire.
This was working last time I used it a few days ago. Find me IG and we can test it. It did show both damages in the combat log. If you try to use the little metal domain icon that looks like a gear it doesn't work. You need to set one of your player tools in the crafting menu to activate the metal domain power then quickslot it.
You'll have to explain to me how to do all of that, PD. That all looks like EFU special scripting code that I'm not familiar with.
Thanks for the help though. :D