But he says he was sure the operation would be investigated "as it should be". Interviewed for BBC Radio 4's World at One programme, Blair said it should be remembered that "people in the Middle East were also trying to fight terrorism and extremism", and that Britain's co-operation with Libya at the time was important.
On air, he referred to comments made by Jack Straw, then foreign secretary, who has said the government was opposed to unlawful rendition. Straw maintained: "We were opposed to unlawful rendition. We were opposed to any use of torture. Not only did we not agree with it, we were not complicit in it and nor did we turn a blind eye to it." But he has also said: "No foreign secretary can know all the details of what its intelligence agencies are doing at any one time."
Blair told the World at One: "Our security services do a very difficult job in very difficult circumstances. I'm sure the matter will be investigated as it should be."
The US is preventing MPs seeing evidence of British involvement in the CIA's practice of secretly sending terror suspects to jails where they could endure torture.
A federal judge in Washington has used part of the US freedom of information act to block a request from Britain's all-party parliamentary group on extraordinary rendition, chaired by the senior Tory backbencher Andrew Tyrie.
The judge, Ricardo Urbina, said that the information had to be withheld on grounds that the parliamentary body was part of a "foreign government entity".
Tony Lloyd, a deputy chair of the committee and Labour MP for Manchester Central, described the ruling as odd. He said it seemed as though the US was looking for an excuse to withhold the information.
It would have been more understandable had the US blocked the request on national security grounds, Lloyd said. "It's an abuse of the spirit of freedom of information," he said. To claim that a parliamentary body was part of the British state was not acceptable, Lloyd added.
Defending the CIA's stance, the judge ruled: "Because the court concludes that the plaintiffs are representatives or subdivisions of a foreign government entity, the court grants the defendants' motion and denies the plaintiffs' motion."
The parliamentary group had requested records that would determine Britain's role in assisting the US by "facilitating such practices, including allowing over-flight or refuelling of planes through or on UK territory or airspace, or by allowing UK territories to be used to hold detainees".
The group referred to statements by the former Labour foreign secretary David Miliband saying Diego Garcia, the US base on Britain's Indian Ocean territory, was used for extraordinary renditions.
The judge rejected the group's argument that its own members were acting as individuals, not public officials. By that logic, the judge said, any foreign leader, could submit Freedom of Information Act requests in an individual capacity.
The CIA's approach echoes that adopted by MI6 and MI5, which have fought to prevent the disclosure in British courts of evidence relating to the US practice of extraordinary rendition.
The parliamentary group, meanwhile, is fighting the British government's refusal to disclose papers that, it says, would reveal complicity in secret flights and subsequent abuse of suspects. The information tribunal in London is expected to rule on the request soon.