Blanchflower served on Labour's economic advisory committee, but quit in June over the party's failure to effectively campaign against Brexit.
And he has now hit Corbyn with another scathing blast, arguing that the party “has nothing to say on economic policy”.
Writing in the Guardian, Blanchflower said: “The UK is in a major economic crisis, caused by the shock Brexit vote, that is being exacerbated by a political crisis. We have a newly appointed chancellor of the exchequer, and an opposition that has nothing to say on economic policy.
“What exactly would John McDonnell do if he was chancellor? He hasn’t told us. Sad to say, as one of his former economic advisers – I was on Labour’s economic advisory committee – I have no idea either. Labour does not seem to have a credible economic plan.”
He added that the economic committee met just twice, and was given the opportunity to contribute to few policies.
“The country needs an opposition with a credible set of worked-out and carefully funded economic policies.
“The UK appears to already be in recession, but the Labour party has nothing to say. There is no plan. That’s not good enough. It’s time it got one,” Blanchflower said.
It comes after Blanchflower and his fellow committee member Oxford economics professor Simon Wren-Lewis backed Smith's leadership bid.
Meanwhile, up to 50,000 Labour supporters could be barred from voting in the upcoming leadership election.
184,000 people applied to become “registered supporters” in a 48 hour window last month in a bid to secure their votes.
However, reports suggest that more than one-in-four of those who signed up could be blocked from voting.
According to the Huffington Post, 40,000 have already been deemed unsuitable, either because they have previously supported rival parties, they are absent from the electoral register or because their £25 payments bounced.
And a further 10,000 are awaiting review by the party's National Executive Committee.
Supporters who paid £25 but blocked from voting will not be entitled to a refund, meaning that Labour could still raise more than £4m in funding from Smith's leadership challenge.