The former premier said there was a "huge stab of anxiety" that the prospect of a British exit from the EU would be on the agenda for the next four or five years.
Blair said the decision to make the promise reminded him of a scene in comedy Blazing Saddles where the sheriff threatened to blow his own brains out.
He told BBC Radio 4's World At One programme: "To do that at this moment is a worry for any of us who have been through the European mill.
"Right now, these other European countries are in a very, very difficult state themselves.
"It reminds me a bit of the Mel Brooks comedy Blazing Saddles where the sheriff... holds a gun to his own head and says 'If you don't do what I want I'll blow my brains out'.
"You want to watch one of the 26 don't just say 'Well, OK, go ahead'."
Blair said he had been able to achieve "significant reforms" to the EU without threatening British exit - and that the lack of such an ultimatum was not to blame "for any shortcomings" in those efforts.
He also told the BBC, Labour leader Ed Miliband was "absolutely right in a sense to say 'at this moment in time what is the point of putting on the agenda the prospect of Britain leaving'."
"If I were Ed Miliband, I would keep my powder completely dry at the moment.
"I would say I'm going to see what Europe proposes, I'm going to see what David Cameron proposes and therefore see what is actually going to happen rising out of the euro crisis," he told Sky News.
EU Twitter Reaction
Blair said he could agree with "large parts" of the speech but dismissed the promise of a referendum as an attempt to placate Ukip and disgruntled Tory backbenchers.
He told Sky News: "It's the climax to the speech that's the 10% that I'm afraid is really about the internal European problem in the Conservative party and how they placate this Ukip party and that is where you put on the agenda the prospect of Britain leaving Europe.
"That's not about our negotiation, that's about putting on the agenda the prospect of a British exit from Europe when, for four or five years, you can't answer the question 'Is Britain going to remain in the European Union or not?'"
Blair added: "Europe does need Britain and Britain needs Europe, which is why the sensible thing to do is to argue the case for reform in Europe.
"But the issue for me is why put that other question, why say we are actually going to put on the agenda the prospect of leaving altogether?
"Why would we do that?
"Why would we do that now when we don't know either what we are proposing, what the rest of Europe's proposing or what the outcome of these negotiations is going to be?"
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