Actually, the real anniversary is tomorrow. Blair did not become Prime Minister until May 2, 1997, the day on which he was invited by the Queen to form a government, but predictably, most newspapers are treating today as marking Blair's 10 years.
I'll give my considered thoughts on what his premiership will be remembered for after he announces his resignation next week, but in the meantime, what significance should we attach to the fact that Blair has now emulated Walpole, North, Pitt, Liverpool and Thatcher by serving a continuous decade in power?
Well, the answer to that is not a lot, in my view. As that list demonstrates, it's a milestone that neither necessarily reflects greatness, nor necessarily confers it.
The truth is that Blair should not have remained Prime Minister this long, either for the good of the country, the good of the Labour Party, or for the good of his own historical reputation. That he has finally chalked up ten years is more a tribute to his tenacity and to the paucity of alternatives than to any real and lasting sense of political achievement.
As I have written before, Blair should in all conscience have gone in 2003, after the David Kelly scandal. The fact that no-one in the government was prepared to take the rap for this tragic episode has always seemed to me an appalling dereliction of responsibility.
Whether or not it was Alastair Campbell himself, it is quite clear that someone in the government spin machine took the decision to release Dr Kelly's name, and under the doctrine of ministerial responsibility, it was Mr Blair who should have been ultimately held accountable.
As the late Hugo Young wrote at the time, the suicide of Dr Kelly was no random act of chance. It was an illustration of "the dynamic that is unleashed when the Prime Minister's sainted reputation becomes the core value his country has to defend."
Blair could even have made it a resignation on a point of honour, like Lord Carrington's over the Falklands invasion in 1982. He could have said "I was not responsible for this, and I deplore the chain of events that led to it, but the buck stops with me."
Of course, Blair survived, but nothing was ever quite the same again. Early in 2004, he seems to have experienced a momentary realisation that the catastrophic loss of trust that had occurred as a result of the war and its aftermath could not be regained under his leadership.
He could have gone then, handed over to Gordon Brown while the latter's reputation was still sky-high, ensuring Labour another three-figure majority in 2005 over Michael Howard's right-wing Tory rabble.
Instead, Blair hugely outstayed his welcome, and the results of that will be plain for all to see in Thursday night's local elections when Labour's support slumps to near the levels the party enjoyed when he first entered Parliament at the "suicide note" election of 1983.
Less than a year ago, a leaked Downing Street memo laughably suggested that Blair should "go with the crowds wanting more." He's actually going when the crowds can't wait to see the back of him. And he has only himself to blame.