User blog comment:Roranoa zoro/Eternal Flame,FT's foundin' members,Acnologia's DSE/@comment-10425110-20130714150934/@comment-7418318-20130714154022

Both Etherion and the united attack of all the members of the Tenrou Team had a similar glow to it, so my guess is that it's roar is a combination of it's own element with all the others Acnologia absorbed from it's victim Dragons.

As for Acno's original element...

So far everytime a new Dragon Slayer's element was revealed Mashima made is something new (as in, no other DS in the story had it), so we can probably rule the established seven out. And crystal is too non-specific I think, since it's a term that encompases many different materials.

Maybe Onyx or Obsidian? That could explain it's dark color.