User blog comment:Roranoa zoro/Eternal Flame,FT's foundin' members,Acnologia's DSE/@comment-10425110-20130714150934/@comment-7418318-20130714154945

Right. I forgot about that line "unlike any Dragon he had black wings"...

Diamond can have many different colors depending on the kind of impurity in it's crystal mesh, so it's teacher might have been white or even translucent, but with Acnologia turning out black with blue stripes.