User blog comment:Sargenttacos115/The side efx of Dragon slayer Magic/@comment-4820738-20131109145241/@comment-7418318-20131109152023

We don't know how much of Acnologia's transformation was due to "bathing in Dragon blood" and how much of it was due to Zeref's magic.

Also just killing Dragons doesn't mean you have to even get into contact with their blood, much less bathe in it.