Yeah I believe that Zeref is the black dragon slayer, but what is more interesting that the gap of years comes into play, and not just a small amount of years, 400years...
Either Natsu being a child that we saw is a hallucination of Natsu or that he's missing years from his memory...
But what strikes odd is that he came to the guild as a child, so maybe a cycle of rebirth is here? Or some magic happened that turned him back into a child? Well mystery mystery...