Tony Blair's decade in power is seared with disappointments – but there is one cool, consistent success story that ran through his time in power: the rapid advance of gay rights. If we had known in 1997 we would achieve full legal equality – even including de facto marriage – so fast and with so little fuss, we would have been startled.

When I interviewed Blair about gay rights last month to mark the 15th anniversary of Attitude magazine, I glimpsed his very best side – and the strange, gaping blind spots that did so much harm to his record, and the world.

Leaning forward, Blair offers a passionate defence of the equality of gay people. He talks about how, from his school days, he had friends who were terrified to come out and how the homophobia of the Conservatives represented "everything I wanted to change" about Britain.

He talks about how political correctness is used by "reactionary forces" as "a cover by people arguing against basic equality. Equality isn't political correctness, it's just justice." He says with a smile that delivering on it was one of his "proudest achievements".

And he transfers this success into an almost Messianic optimism about the future. As probably the most high-profile pro-gay religious person in the world, he says he is "optimistic" that all religions – including Islam – can go through "a process of Reformation" that will end with them accepting openly gay people. It is part of the "mission" of his Faith Foundation to move religion away from anti-gay literalism.

He doesn't hide his disagreement with the anti-gay bile of the leader of his own faith, the Pope. He says "there is a huge generational difference here" and that "if you went and asked the [ordinary Catholic] congregation, I think you'd find that their faith is not to be found in those types of entrenched attitudes." The fight for gay equality was a rare occasion when Blair took on the Right. I ask him if he wishes he had done it more, and he looks thoughtful. "It depends on the issue. But yes."

And yet, and yet... I soon crash into the blind spot that sent his premiership spinning to an early death. I ask him if he ever discussed his pro-gay views with George Bush. "No, I can't say I did. I mean, here's an interesting thing. I honestly haven't the faintest idea of how he voted on any of these things, but I'd be quite surprised if he personally were prejudiced." It's a bizarre answer. Of course he knows what George Bush did to oppose gay equality – he reads the newspapers. Why not just say that he disagrees? Why lie (and add the word "honestly" as you do it)? Why actually defend a man whose views on gay people are so obnoxious, and so opposite to his own?

Wrapped into this little interview was the paradox – and the tragedy – of Tony Blair. When he chose to fight on liberal issues, he was passionate, and brilliant. But he did it only a few times – and he willingly suspended these, his most impressive and admirable instincts, to embark on a bloody barn dance with the worst president in living memory. Why?