Knave or fool?
"I did not have sex with that woman, Miss Lewinsky."

Politicians (ahem) fail to tell the truth all the time. The issue with Blair is the enormity of the consequences of his utterances over Iraq.

I once had to apologise to an OFSTED inspector who had made a blatantly untrue statement because I could not prove that he knew any better. I had to concede that he might not be lying, he might be unbelievably ignorant.

Likewise we cannot prove that Blair didn't actually believe that Iraq had Weapons of Mass destruction which posed an immediate threat. Perhaps he thought that they had these weapons which they would use as a last resort if invaded so the best thing to do was to invade them.

So either a liar or criminally insane