At 6-foot and 210 pounds, Blackmon last played in the NFL for the New York Giants in 2011, appearing in six games and finishing with five tackles.
The 28-year-old cornerback spent 2012 with the Arizona Rattlers in the Arena Football League.
A fourth round selection by Green Bay in the 2006 draft, Seattle general manager John Schneider was with the Packers when they selected Blackmon, so he has some familiarity with the Boston College product’s skill set.
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Blackmon has experience playing inside on the slot receiver in nickel situations.
Blackmon also returned kicks and punts while in the NFL. He averaged 20.7 yards a kick return, with a long of 45 yards on 81 returns. And Blackmon also averaged 9.5 yards per punt return on 76 attempts, including three for touchdowns.