Hamid Reza Asefi, a foreign ministry spokesman, said: "It would be good for Mr Blair to participate in the Holocaust seminar in Tehran.
"He can also contribute with an article. If he wants to defend the Holocaust in that article, he can do so. We will give him the time to read out his article so others can hear his point of view."
The conference is scheduled for spring.
On Monday, Tony Blair attacked the meeting, saying it was "shocking, ridiculous, stupid". He said Ahmadinejad "should come and see the evidence of the Holocaust himself in the countries of Europe".
Asefi went on: "In Mr Blair's speech there was an invitation for the Iranian president to go there and see the places close up. We have to see when the president has time for it."
Iran's foreign ministry has already said it is willing to send a team of independent investigators to visit former Nazi death camps across Europe.
Ahmadinejad, a populist conservative who came to power in a surprise victory last June, has provoked international condemnation with a number of anti-Israeli remarks.
They include labelling Israel a "tumour" that should be "wiped off the map" or moved as far away as Alaska and saying the Holocaust - the systematic slaughter by the Nazis of mainland Europe's Jews as well as other groups during the second world war - was a Western invention.
Iran has presented the planned conference as an exercise in free speech, where Asefi said Blair "can say the kind of things he cannot say in London".