The Seahawks have signed 28-year old kick returner and defensive back Will Blackmon. Blackmon was most recently with the Arizona Rattlers of the Arena Football League, and before that, played on the 2011 New York Giants Super Bowl Champion team.
Blackmon was drafted by John Schneider's Packers in 2005 in the 4th round, out of Boston College, after making a name for himself by spending his first three years at BC as a cornerback and then making the switch to WR his senior year, faring surprisingly well. Blackmon handled a lot of return and special teams duties with the Packers before he tore his ACL in 2009, prompting an injury settlement release in training camp 2010. He signed with the Giants, was placed on IR, released, and then re-joined the team mid-season in 2011 and got a ring with them as they beat the Patriots in the Super Bowl.
Blackmon has averaged 20.7 yards per kick return during his career, as Eric Williams points out, with a long of 45 yards on 81 returns, and he averaged 9.5 yards per punt return on 76 attempts, with three run back for touchdowns.
Seattle worked Blackmon out during the season last year but didn't sign him, which is fairly typical for this front office, as they build a board of street free agents that they have interest in. Blackmon is now three years removed from that reconstructive ACL surgery, it's probably safe to say that full strength has returned there and his quickness should no longer be affected. Prior to his injury, the Boston College product was one of the league's elite athletes - just going off of the numbers we do have from the Combine back in 2005, at 6'1, 200, he ran the 40 in 4.46 (would have been 6th among DBs in 2012), the 3-cone in 6.69 (would have been 5th), and had a 41" vertical and an 11 foot broadjump, both of which would have been first in 2012 among DBs. So, obviously, he had/has a very explosive element of athleticism to his game, assuming he's back to anywhere near 100%.
At 28, he's still young enough to perform at near-peak levels and my guess is that he's been training hard over the last few months, with the expectation he'd get another call from Seattle (which has now happened, which makes me think he's in excellent shape). Judging by his history as a prolific return man in college and respectable numbers in the NFL, that's where the Seahawks plan to use him, if he makes the team, and he also provides depth in the secondary, as it's noted that he has experience at both the outside cornerback spot and in the slot as a nickleback.
Blackmon probably faces long odds of making the final roster in 2013 - Seattle still has a bevy of draft picks and the DB position is already stacked, but you never know -- if Leon Washington becomes expendable due to his salary or if the Seahawks look to move him, they'll need a kick returner to take his spot. I imagine this is just an effort to build in a little depth during the offseason.