Lukewarm take: both can be true. Blair is a terrible human who started a war on false grounds that killed over a hundred thousand people, destabilised an already volatile area and laid the foundations for terror and violence ever since. He knew that there was no reason to invade and still went ahead, and now the world is much worse off. He has paid basically nothing for this himself. Fuck him forever for that.
BUT he's not actually wrong about Brexit. Stopped clock and all that. Cameron wasn't wrong either - he made it harder to vote Remain because it meant voting for him! But as wrong as both of those twats are generally, and as unfit for office and all that, they are actually in the right this time. Not that people should listen to them - there are much better people arguing for remain/single market and so on - but in this particular case, they're not wrong. But still, papers etc should be ignoring them, Blair especially, on principle.