I can't cast Darkfire on elemental summons. It works on the vermin and animal, but not elemental. Flame weapon works on elemental summons tho, so makes me think this is a bug.

Tested this in game and it works as intended.

Some things to remember - if used on a fire type elemental your casting level as to be high enough to be better than the summons natural ability, or you have to use the book of transmutation to change elemental types. Also, the elemental summons have three weapons (bite/claw/claw) and the Darkfire spell is only applied to one of those weapons, so you won’t see it in the combat log until it lands a hit with that particular creature weapon. Lastly, it does not show up on the creature’s character sheet, but you will see it in the combat log.