His role was to change from being a politician to being a diplomat. It his impossible to access whether he has been a success or a failure. We do not know the state of things in the middle east if he hadn't have been given the job, they might have been worse. Equally be do not know if anyone else had been given the job, whether things would have progressed better.
I do not like Blair, and believe he has done a terrific amount of harm to the UK and other countries, but on his middle east role, it is too early and a mistake to call it a failure.