On September 15, 2011, the 42-year-old Royal Marine shot a Taliban fighter who had been seriously injured by fire from an Apache helicopter in Helmand province.
"There you are. Shuffle off this mortal coil ... It's nothing you wouldn't do to us," said Blackman, paraphrasing Shakespeare's "Hamlet" after shooting the fighter at close range with a 9mm pistol.
"Obviously this doesn't go anywhere, fellas. I just broke the Geneva Convention," he said to his fellow Royal Marines, in reference to international law governing the treatment of prisoners of war.
Footage of the incident was captured on a camera mounted onto the helmet of one of the soldiers, and two years later Blackman was convicted of murder by a court martial.
The case was the first time since the Second World War that a British soldier was convicted of a murder carried out on the battlefield.
"You treated that Afghan man with contempt and murdered him in cold blood," judge Jeff Blackett told Blackman in December 2013 as the soldier was given a life sentence with a 10-year minimum.
"By doing so you have betrayed your corps and all British service personnel who have served in Afghanistan, and you have tarnished their reputation," Blackett added.
The conviction was supported by Britain's then chief of defence staff, Nicholas Houghton, who said: "Murder is murder, this is a heinous crime."
But now Blackman is making a bid for freedom, with a court hearing in London on Wednesday set to decide if he will be granted bail.
'Icing on the cake'
The soldier's legal team ultimately aims to overturn the conviction and have the crime lessened to manslaughter, following a Royal Navy review which suggests Blackman was suffering from post-traumatic stress at the time of the shooting.
Blackman has denied murder and claimed he shot the Taliban fighter once the insurgent was already dead.
His wife, Claire Blackman, told the BBC she hoped to have her husband home for Christmas.
"Bail would be the icing on the cake," she said.
Dozens of veterans are due to gather on Wednesday outside the Royal Courts of Justice to call for "Justice for Marine A, abandoned by the government," according to a poster.
Blackman's supporters include Richard Kemp, a former commander in Afghanistan, who said the soldier had been "thrown to the wolves by cowards".
MP Richard Draw and author Frederick Forsyth have promised to pay the £200,000 bail.
The Daily Mail newspaper has also campaigned against the murder conviction.
"Did a man who for 13 years risked his life defending his country, often in the most appalling environment... really deserve to be tried for murder and thrown in jail like a common criminal?" the right-wing newspaper wrote.
Beyond the "Marine A" case, British tabloids have campaigned against former soldiers facing trial for alleged crimes committed while on duty.
They received support from Prime Minister Theresa May, who in October promised to "put an end to the industry of vexatious claims that has pursued those who served in previous conflicts".