User blog comment:Inept Wiki User/Speculation: Acnologia and Assimilation/@comment-4488668-20130522174815/@comment-7418318-20130522175812

Didn't Zirconis' Ghost say "unlike any Dragon he had black wings" or something like that?

If I remember that correctly, it would mean that there are no black Dragons and thus he couldn't have started out as a Black Dragon Slayer.