Personally, I don't feel particularly qualified to make a judgement with regard to how much of a threat Iran is (and with all due respect I'm not particularly convinced my fellow ABers are either). While it's not one I subscribe to or am particularly interested in, I can also see the moral argument in favour of intervention.
The most important factor in all this to me seems to be that the country would quite simply not accept it. If you hear people talk about Afghanistan, the way people on average appear to perceive the military situation there is that we're hanging on by our fingernails and constantly losing people in a futile effort to keep control. People also seem to view the country's finances in a similar way. Despite the fact that people are extremely scared of Islamic powers, involving ourselves in such an intervention would severely destabilise our society.
Blair's suggestion is thus highly questionable considering his position, and is not remotely grounded in practical reality. He appears to have (just as implied in the case of Iraq) permanently lost the shrewd insight into domestic politics he demonstrated earlier in his career.