The former prime minister will be able to say that the action he took was, as he told the House of Commons when it supported his action, legal because he had been given advice to that effect by his government's most senior counsel in these matters, the attorney general, Lord Goldsmith.
Whatever one's view on the Iraq conflict, and it has bitterly divided opinion in Britain, this is the inescapable and, for some, inconvenient truth that lies at the heart of the controversies over Mr Blair's obvious desire to lead this country into war.
Yet despite Lord Goldsmith appearing to give the former prime minister the defence of veritas, there are still a number of questions that arise out of the former attorney General's evidence yesterday.
The first is over his decision to change his mind on the legality of war. Lord Goldsmith told the inquiry that throughout the build-up to the invasion of Iraq he continued to believe that military action would be unlawful without a second United Nations Security Council resolution on the issue.
The first Security Council resolution, 1441, requiring Iraq to give up its supposed weapons of mass destruction, had been negotiated with France and Russia by Britain and others and did not, Lord Goldsmith believed at the time, make military action legal.
However, after a meeting in Washington with senior US officials and lawyers – including national security adviser Condoleezza Rice and senior State Department legal adviser Will Taft – Lord Goldsmith promptly changed his mind.
Yesterday's session of the inquiry failed to probe further into this event. Was Lord Goldsmith given legal papers? Was he simply told the US believed the action was legal?
We do not know – which is a highly unsatisfactory state of affairs.
The second point that was not fully explored was the amount of influence Downing Street had over this decision, although Lord Goldsmith denied reports that he had been pressurised by members of Mr Blair's staff into changing his mind.
Thirdly, both the former attorney general and Lord Chilcot himself yesterday expressed frustration that papers setting out, over time, Lord Goldsmith's views – views apparently challenged by the then foreign secretary Jack Straw – will have to remain secret.
As we approach Mr Blair's appearance in front of the inquiry, this latest session has shed some light into the dark corners of the corridors of power but left too many important facts hidden in the shadows, obscuring the truth.
It is to be hoped that the Chilcot inquiry will pursue these issues assiduously and with vigour when Mr Blair, the man who led us into war, appears before it tomorrow.