Mr Blakelock, 40, was attacked by an armed mob after his unit was deployed to protect firefighters tackling a blaze on the Broadwater Farm estate in London on the night of October 6, 1985.
The Old Bailey trial heard the officer was stabbed more than 40 times and that rioters tried to decapitate him.
Nicky Jacobs, 16 at the time, was the seventh person to be charged with the murder but no-one has been found guilty.
When the verdict was delivered, Mr Blakelock's widow, Elizabeth Johnson, left the courtroom and wept, while her three sons held their heads in their hands.
The family issued a statement saying they were extremely sad and disappointed.
Mr Jacobs punched the air and sobbed as supporters let out shouts of joy.
Supporter Stafford Scott said afterwards questions should be raised with the Crown Prosecution Service and Director of Public Prosecutions as to why such a "farcical" case came to trial.
The Crown Prosecution Service said that, while it accepted the verdict, it was right to bring the case.
Scotland Yard Assistant Commissioner Mark Rowley vowed: "We will not give up on bringing Keith's killers to justice."
Mr Jacobs's prosecution followed a trial in 1987, when three men were convicted of the murder but freed on appeal. The case against three youths was dropped.
During the course of three separate probes, a decision was made to give immunity to so-called "kickers" - those who were involved in the attack but did not use weapons - in exchange for their co-operation.
Among them were prosecution witnesses given the pseudonyms Rhodes Levin and John Brown, who both admitted kicking Mr Blakelock and were given amnesty from prosecution.
Some of the witnesses also received payments from police for their co-operation, the jury was told.