The commando, who spent more than three years in jail for killing the insurgent in 2011, left Erlestoke Prison just before 12.20am on Friday.
Blackman hid from cameras as he was driven away from the prison near Devizes, Wiltshire, in the back of a dark Audi estate.
He was originally convicted of murder but this was quashed by the Court Martial Appeal Court and replaced with diminished responsibility manslaughter last month.
Five judges ruled that Blackman was suffering from an "abnormality of mental functioning" at the time of the incident, when he was serving with Plymouth-based 42 Commando.
His sentence was reduced to seven years, meaning he would be freed within weeks - a decision his wife Claire Blackman described as "the moment that we have all been fighting hard for".
Speaking to the Daily Mail while awaiting her husband's release, she said: "I haven't slept all week. I feel like a child waiting for Christmas. We have had so many setbacks over the years that I almost cannot believe it is really going to happen this time.
"I can't wait to see him and spend some quality time together."
Blackman was convicted of murder at Bulford Court Martial in 2013 and jailed for life, with a minimum term of 10 years.
This sentence was later reduced to eight years.
His wife Claire led a campaign for Blackman to be released, which was taken up by the Daily Mail.
In March, his conviction for murder was quashed and replaced with manslaughter on the grounds of diminished responsibility.
During the original trial in 2013, Blackman was known only as Marine A.
His identity was made public after his conviction.