Acnologia's Element?

I've read a lot of stories and fanfictions about Acnologia's element. Some say that he's a Black Dragon. Think about it. If there was a Black Dragon Slayer, what would he eat? Black? And if he's a Death Dragon, then the Death Dragon Slayer would eat .. death? That doesn't make any sense lol. Sorry I'm just curious. I'. not professional when it comes to Fairy Tail.