Blalock, 31, was Atlanta's second-round selection (39th overall) out of Texas in 2007. He started all 125 games he played for the Falcons, including 15 last season. Between 2007 and 2014, he started 102 consecutive games.
"I'd like to sincerely thank the Atlanta Falcons, all my teammates and especially the fans," Blalock wrote on Twitter. "I've had an unbelievable eight years and only wish I could have helped bring a championship to the ATL."
The Falcons also announced the release of wide receiver Harry Douglas on Friday after cutting running back Steven Jackson earlier this week. The moves come in wake of a disappointing 6-10 season, after which Atlanta fired head coach Mike Smith.
The team hired Seahawks defensive coordinator Dan Quinn as Smith's replacement earlier this month.
The 6-foot-4, 326-pound Blalock would have made 4.75 million this season.