Blair, now a Middle East envoy, reflected on his relationship with Gaddafi during an interview with Reuters Insider marking the 10th anniversary of the 11 September Al-Qaeda attacks on US cities.
He said he could not be sure that Arab countries swept by popular revolts would move to democracy, citing a risk that hardline elements hostile to Western interests could triumph.
Gaddafi's whereabouts are unknown after he was ousted two weeks ago by an armed revolt coupled with NATO bombing raids in which British warplanes played a leading part.
Asked if he regretted putting aside years of British hostility and holding out an olive branch to Gaddafi in 2004, Blair said: "No. I always say to people it is absolutely simple -- the external policy of Libya changed."
Libya had long been an international pariah before Blair flew to Tripoli in March 2004 to seal the country's return to the world community with a historic handshake with Gaddafi.
He did so after Tripoli announced it would abandon efforts to acquire chemical, biological and nuclear weapons, a step Blair said was "a great thing for the world," and cooperate in the fight against terrorism.
Libya had earlier agreed to pay damages for a 1988 airliner bombing over Lockerbie, Scotland which killed 270 people.
"The trouble was in the end they weren't prepared to reform internally ... They were less of a threat to the outside world, but inside they were a threat to their people and then when the uprising happened, again, there was a big choice," Blair said.
"I remember actually speaking to Colonel Gaddafi at the time (the uprising) happened and saying this is the moment to realize you are going to have to go and be the person that gives it up," said Blair, who was prime minister for 10 years from 1997.
Blair confidant Peter Mandelson said in February Blair had called Gaddafi urging him to step down. The plea fell on deaf ears.
Blair returned to Libya in 2007 when he was pictured smiling and shaking hands with Gaddafi and holding talks with him in a tent in the desert near the Libyan strongman's home town, Sirte.
Those images have come back to haunt Blair as reports of the brutality of Gaddafi's government have emerged.
During Blair's 2007 visit, British oil firm BP signed a big natural gas exploration deal with Libya, raising questions about whether commercial interests influenced British policy.
Britain and Libya had fraught relations for much of Gaddafi's 42-year rule. A British policewoman was killed by a shot fired from the Libyan embassy in London during an anti-Gaddafi protest in 1984.
In 2009, two years after Blair left office, the Scottish government released cancer-stricken convicted Lockerbie bomber Abdel Basset al-Megrahi on compassionate grounds, angering the US government and relatives of the 189 Americans killed.
Blair was concerned that the Arab Spring could open the door to hardliners. "Those people who are out there protesting may well be the majority but are usually not really very well organized. The (Islamist) Muslim Brotherhood and others are very well organized," he said.
When the first airliner hijacked by al Qaeda militants smashed into the World Trade Center in New York 10 years ago, Blair was in a hotel in Brighton, England, preparing to address a trade union conference.
"I was then interrupted by one of my aides who said 'Come through, you've got to see what's happening on the television'. The first plane had already flown in and hit the tower," Blair said.
"I was actually very, very clear right from the very outset that this was not just a terrorist attack of an extraordinary magnitude but one that had to change global policy," he said.
"What was clear was that were those terrorists able to get hold of technology to kill even more people, they would. That was where everything to do with nuclear and chemical weapons and anxieties about those stemmed from," he said.
Blair, now 58, stood shoulder to shoulder with President George W. Bush as he launched a "war on terror," sending British troops to support US-led invasions in Afghanistan in 2001 and, more controversially, Iraq in 2003.
Blair and Bush accused then-Iraqi President Saddam Hussein of having weapons of mass destruction, but none were found.
Ten years later, Blair said, Al-Qaeda still posed a risk even though the West had "degraded a lot of their capacity."
The killing of Al-Qaeda leader Osama bin Laden in a US commando raid on his hideout in Pakistan in May this year had dealt a "huge psychological blow" to the group but many people around the world still share the movement's ideology, if not its methods. "That's what's got to be defeated," he said.
Blair said he did not anticipate that, almost 10 years since the invasion of Afghanistan, Western troops would still be locked in fierce fighting with the Taliban.
"I thought, as most people did, that if we knocked out the Taliban, gave people a democratic process, gave them large sums of money to reconstruct the country, we could reconstruct it. But this is what shows you these forces are very deep," he said.
Blair gave the interview on 2 September but it was held for publication until now at the request of his office.