User blog comment:IamJakuhoRaikoben/Chapter 449: Mavis & Zeref, Review./@comment-26389992-20150824122515/@comment-9595671-20150827234208

I agree, Mavis called it Fairy Tail's "Light", and even Precht referred to it as such during the Tartaros arc.

Ivan is the only one who's referred to it as their Darkness, and its likely that was just cause Precht had been running Grimoire Heart at the time.

Getting info about something like that from a Dark Guild wouldn't make it sounds like it's a good thing at first thought since most Dark guilds do worship Zeref after all.