North Yorkshire and Merseyside council pension funds all alledge that that Sir Fred Goodwin and RBS ‘falsely reassured' investors about the financial state of the bank.
They have hired the ex-PM's wife to represent then in the case submitted to a New York court.
Blair will work alongside US-law firm Coughlin Stoia Geller Rudman & Robbins, for which Byfield is does media relations work in the UK. The agency has been behind the flurry of media interest around the case and broke the story on the front page on the Times on Monday.
The agency will give ongoing media support as the case progresses.
The US law firm recently secured US$7bn in compensation for Enron investors - a case that Byfield also publicised in the UK.