Well, Blitz is one of the original champions from the game's release, I do believe. Back then, champions were not made for a specific meta. Blitz was likely originally made to be capable at solo laning, but as the meta found its place in the game (1 Top, 1 Mid, 2 Bot, 1 Jungle), it turned out Blitz had a lot of Supportive capabilities, so he gradually moved into the Support position, where he's pretty much only ever seen now.
Because he has a pretty straightforward kit (land grab for a kill) that both teams can play around, he's not really seen any major changes.
As for why he has the AoE damage on his ult? I guess with the lack of major changes to his kit, he has it because of the original intent of him being a fighter/tank in a solo lane. He's in a decent spot as far as having a kit that works together, just that it doesn't exactly fit the role he's come to be known for.