The former Prime Minister suggested that he did not want to be drawn on the commitment.
It follows a warning from former Business Secretary Peter Mandelson that the pledge could set back Labour's image.
Asked about Mr Miliband's speech, Mr Blair said: "I'm not really going to comment on Ed's conference speech. It seemed to go down very well with people and was excellently delivered, I think. But I'm not going to comment on the policy.
"He's got the job of being leader of the Opposition. I did that job for three years, I know how tough it is, I'm not going to get in his way."
The pledge to cap energy gas and electricity prices for 20 months was the centrepiece of Mr Miliband's speech. His aides insist he is happy to take the fight to the energy companies and be seen, in contrast, they say, to David Cameron, as on the side of ordinary people.
But the row that the policy has created within the Labour party now threatens to overshadow that with big business. Lord Mandelson criticised the plans saying: "I believe that perceptions of Labour policy are in danger of being taken backwards."
But Blairites, including the former prime minister's one-time spin chief Alastair Campbell, lined up to back Mr Miliband.
The policy announcement has been blamed for wiping £2 billion off the value of the UK's two largest power firms. Energy companies and Labour's political opponents have also raised the spectre of energy blackouts triggered by the plans.