Nicholas Jacobs, 45, was found not guilty on Wednesday of the murder and manslaughter of the officer who was stabbed 43 times at Broadwater Farm.
But he was kept in prison overnight as administrative offices had closed.
His solicitor Tony Meisels said his client was "relieved" to be free but his "thoughts are with PC Blakelock's widow and her children".
Outside Belmarsh Prison Mr Meisels said: "He told me he slept like a baby. He knew he was getting out this morning.
"It is one more thing the authorities have thrown at him."
Earlier Mr Meisels criticised the authorities for keeping Mr Jacobs in prison the extra night and said it was "almost like the last small laugh of the police."
'Extremely unusual'Following the verdict Mr Jacobs was returned to Belmarsh Prison for the relevant paperwork to be carried out, but Mr Meisels said, despite ample warning, the officers involved went home.
He said: "It is extremely unusual and I've never come across it before and it's a shame.
"The comment I was told from one of the senior prison officers was that 'this case is of a high media profile and we don't want to release him in error'.
"He doesn't get the experience of walking out the front doors of the Old Bailey."
In a statement, the Ministry of Justice said it did not comment on individual cases.
It added: "Public safety is our priority and prisons must be satisfied there are no outstanding legal issues before releasing an acquitted prisoner."
Robert Hall looks at what happened on the night PC Blakelock diedPC Blakelock was attacked on the night of 6 October 1985 while protecting firefighters tackling blazes started during the riots.
He was repeatedly stabbed and attempts were made to decapitate him.
The jury at Mr Jacobs' trial heard from three witnesses who claimed they saw the then 16-year-old take part in the attack, but his defence team questioned their credibility.
Outside the prison Mr Meisels also criticised the police's use of evidence from "kickers" - witnesses in the trial who admitted attacking PC Blakelock but were given immunity if they testified.
'Never get justice'He said. "So desperate were the police to secure a conviction that they went to great lengths to alter the rules so they could proceed with the case."
Mr Jacobs was the seventh person to be charged over PC Blakelock's death, but no-one has been successfully prosecuted for the killing.
In 1987 three men were convicted of the murder, before being freed four years later on appeal.
Retired policeman Richard Coombes, 63, who survived the Broadwater Farm attack, said: "We're very disappointed from the point of view of it doesn't give anybody the opportunity to draw a line under this and say well at least somebody's been found guilty of Keith's murder.
"I feel dreadfully, dreadfully sorry for the Blakelock family because they sat through every day of that trial and they must today be feeling that they'll never get justice for Keith."
'Get anyone'PC Blakelock's family said in a statement issued on Wednesday: "We viewed this trial as an opportunity to see some form of justice served for Keith.
"We hope that more people are able to come forward so that some of those guilty can be brought to justice in the future."
Metropolitan Police Commissioner Sir Bernard Hogan-Howe said: "We accept the decision of the jury and our work to bring those responsible for Keith's murder to justice will not stop."
Meanwhile Winston Silcott, who had his 1987 conviction for murdering PC Blakelock overturned, said: "The police are bitter about what happened, that's why they brought this case.
"My conclusion is that they had promised the Blakelock family that they would get justice for him and so they were trying to get anyone they could."