His arrival followed a potential crisis generated by the Ulster Unionists, who earlier in the day delivered a strongly-worded rejection of a working paper tabled by the talks chairman, former US Senator George Mitchell.
Mr Blair met David Trimble, the Ulster Unionist Party leader, for what will probably be the first of a series of such meetings as the talks move towards their deadline of midnight tomorrow. The UUP left the talks for a time but did not threaten a complete withdrawal.
Opinions differed among the other parties involved on whether the chairman's paper had caused a genuine crisis for the UUP, or whether its rejection amounted to a stratagem aimed at extracting concessions in the last days of the talks.
At a news conference on his arrival, the Prime Minister delivered a message which seemed to suggest Unionists would be expected to make concessions in exchange for Northern Ireland's place in the Union.
He said: "The central point that the Unionists have made all the way through is that Northern Ireland should remain part of the UK so long as a majority of the people here want it to be so. The question is that, in return for that, obviously there has to be recognition of the fact that there is a nationalist aspiration and identity there, and we've got to put in place arrangements which make sense of that."
Translated into the details of negotiations, this was taken as an unmistakable sign that the proposed north-south body linking Belfast and Dublin is envisaged as an institution of some substance.
The aim of the UUP yesterday appeared to be to water down the role and functions of this body, as set out in Mr Mitchell's document. The document itself is not a plan, but rather largely a synthesis of proposals already advanced in the talks.
The party accused the Irish government of negotiating in bad faith, claiming that "all relevant accepted international norms have been ignored". It warned that the party could not recommend the document to the people of Northern Ireland for approval, thus in effect warning that it would advise its supporters to vote against any such ideas in a referendum.
The scene has therefore been set for a period of intensive negotiations as Mr Blair and the Irish Prime Minister, Bertie Ahern, attempt to propel the talks parties towards a common position. Mr Ahern's arrival in Belfast was delayed by funeral arrangements for his mother, who died earlier this week.
The Ulster Unionist MP Jeffrey Donaldson declared: "This is not a stunt. It is for real. If the Prime Minister wants to see agreement he is going to have to do a lot better than put forward these proposals."
He accused the Irish government of being intransigent and uncompromising, adding: "There is no doubt that the shadow of Sinn Fein lurks in the background." The party's deputy leader John Taylor was more succinct, commenting: "I wouldn't touch this paper with a 40-foot barge pole."
Mr Blair's spokesman said that the Prime Minister had decided to go to Belfast before Mr Trimble telephoned him yesterday to complain about the working paper. He said of the working paper: "No one has been asked to accept the Mitchell document. It is the basis for negotiations and those negotiations are continuing."
Both Sinn Fein and the SDLP said they were disappointed with the Unionist reaction.
And there was a perception at Westminster that Mr Blair is determined to show to the world that he had been prepared to go the extra mile for a peace settlement, and that no blame for failure can be laid at the Government's door.
Mr Blair is expected to be in Belfast until the deadline tomorrow, missing the final session of Prime Minister's Questions in the Commons before the Easter recess.