Blakemore topped a shortlist also featuring Michael Simkins for The Rules of Acting, and Philip Ziegler for Olivier.
Now in its seventh year, the prize is awarded for the best biography, autobiography or diary in theatre or show business published in the preceding calendar year.
Blakemore's book deals with the turbulent transition period of the National Theatre between Olivier's Old Vic company and Peter Hall's on the South Bank. Blakemore was himself an associate during Hall's regime, but walked out in 1976 after a dramatic falling out.
"I'm in a sense relieved," Blakemore said today after receiving the award at the Garrick Club. "It means that people have accepted that I did make an effort to be even-handed, and that they accept the story as written from my point of view."
Chaired by Sheridan Morley's widow Ruth Leon, the £2,000 prize is funded by small donations from individuals and support from the Garrick Club.
Leon said today: "In another strong year for theatre biographies, any of the shortlisted books would have been worthy winners. Michael Blakemore's Stage Blood was the stand out for us, as it has the immediacy of a thriller, combined with the first person account of an extraordinary moment in theatre history."
Accepting the award, Blakemore revealed he had once worked as Sheridan Morley's boyhood tutor, whilst employed by his actor father Robert Morley.