The Republic of Ireland international made his debut against Scunthorpe in a League Cup tie in October 1991.
"I don't think you will see a player make 500 appearances for his club again," Blackwell said on club's official website.
"With the Bosman ruling coming in it will be such a rarity and before many players before were happy to settle and stay at a club because you couldn't move for much more money anyway.
"It's taken Kelly 14-and-a-half years to get here and I can't see anyone staying at a club that long again, so we may be saluting one of the last bastions of football."