Yeah, I'm with Kraco on the God Slayer magic. It just seems like Black Elemental Magic is inedible to Dragon Slayers, so they act like it's more powerful.
We've never seen a God Slayer be trained by a god, or use it to kill a god(although, to be fair, we still haven't seen a Dragon Slayer kill a Dragon yet).
Disagree. I think it's a stupid and deus ex machina explanation. There's no reason why using up all your power should suddenly allow you to absorb a new power.
A Dragon Slayer killing a dragon and taking it's power would make way more sense.