Somebody asked for it, so here it is:
Darkfire
A flickering halo of purple light surrounds the target, making it easier to hit.
Encounter
Minor Action Ranged 10
Target: One creature
Attack: Intelligence +4 vs. Reflex, Wisdom +4 vs. Reflex, or Charisma +4 vs. Reflex
Increase to +6 bonus at 11th level and +8 bonus at 21st level.
Hit: Until the end of your next turn, all attacks against the target have combat advantage, and the target cannot benefit from invisibility or concealment.
Special: When you create your character, choose Intelligence, Wisdom, or Charisma as the ability score you use when making attack rolls with this power. This choice remains throughout your character’s life and does not change the power’s other effects.
It's important, I think, to note the flavor text in particular. This is a power that makes purplish light... not necessarily fire. So, as a DM, I might balk at this too.
Of course, his 'it only targets creatures' justification is an incredibly lame one, because all powers ever target only creatures. If he were to stand by that ruling, then you could never use any power on anything that wasn't a 'creature', an 'enemy' or an 'ally', the three terms that define targets in 95% of all the game's powers (very rarely, some specifically target objects). That obviously doesn't make any sense... I imagine hilarious scenes in which players try depserately to convince the DM that, no, that sealed chest really is an enemy! It's getting in our way, therefore it's an antagonist, therefore it's an enemy!
What I think is really going on here: the DM sees that the power can't do what the player wants it do. He needs to convince the player of that. Maybe he knows the player is a very literal-minded person. To him, a power that says "Darkfire" should make fire, period. So he needs an overly-literal argument to counter-act that.
Still, I don't think this was a good call, and I don't think this is the way most 4e games are played. What I would've done is say: "Sure, but Darkfire doesn't usually make actual flame. You'd need a successful Arcana check to modify your spell."
The player then rolls Arcana. Since I know he's playing a Drow Ranger, he's very likely to have crappy Arcana, so this is pretty darn likely to fall. Therefore, the DM has the benefit of saying "Yes" and letting his players get creative with their abilities, even when he is for all intents and purposes really saying "No." That's how I'd of handled it, personally. Not saying that's the best solution.