User blog comment:PUNCHURPREGNANT/Gray and Silver Predictions/@comment-59.93.129.56-20140113095710/@comment-7418318-20140113105837

The rumors/theories by Zirconis and Arcadios state that Acnologia turned into a Dragon partly due to bathing in the blood of countless slayn Dragons and partly due to Zeref's meddling.

It was never said, or even implyed that Acnologia turned into a Dragon just from using Dragon Slayer Magic.