There are many rumours that Zeref could be a Black Dragon Slayer of the Black Dragon and it might be true! When I was reading Chapter 209, I noticed that Zeref says "You have grown up, Natsu." So, he must have met Natsu before when he was small (when was with Igneel). It could mean that when Igneel and the black dragon met, Zeref and Natsu met. And also seeing how Natsu's scarf, which was given by Igneel, saved him from Zeref's magic... I think you understand. I thought about this for a long time but this could be real (if Mashima wants it to be like this of course). Tell me what you think about this.