User blog comment:Miskos3/Fairy Tail 537: The Power of Life, Review/@comment-26184007-20170526151859/@comment-7418318-20170528015802

Initially Natsu could eat neither Zancrow's God Flames nor Jackals Curse Explosions, but eventually he always figured out a "trick" or a way to do it anyway.

This shows us the differences and also the similarities between the three. While they are definitely different (hence Natsu's inability to eat them right away like he would regular magic) both God Flames and Curses are still closely enough related to magic for Natsu to eat them anyway, after some trying.

If Natsu can bridge the gap between magic and curse, I'm sure Acnologia would also be able to do it eventually after taking a few hits.