i was just thinking that maybe the black dragon is evil and zeref could be a black dragon slayer. Zeref could be a black dragon slayer and use the ultimate black magic. an ice dragon would have invincible ice magic, fire with fire, and black with black. no fire is greater than that of a dragon because a dragon or dragionslayer will eat that fire and have an even greater fire and the same for a black dragon slayer which could be zeref.
gildarts also said the black dragon probably hated humans and the dragons disappearance is related to zeref and since if he was the greatest black or dark mage he could be a dark dragon slayer