User blog comment:PUNCHURPREGNANT/Gray and Silver Predictions/@comment-69.255.250.225-20140223024513/@comment-1478718-20140223030938

Actually, Acnologia (the Black Dragon of the Apocalypse) was originally a Dragon Slayer. Supposedly, he bathed in the blood of too many dragons (to absorb their power) and Zeref cast some weird spell on his and he became a dragon.