The reason for the suspension has not yet been reported, but unless Blackmon has failed a previous drug test and was already in the leagues drug program, it may wind up being for violating the league's rules on performance enhancing drugs. As mentioned however, it's not clear the purpose of the suspension just yet.
It's possible the Jaguars knew this was coming however, as they surprisingly drafted a wide receiver early in the draft by picking Ace Sanders in the fourth-round. Losing Blackmon however is going to hurt the offense going forward, as he appeared to be emerging as the Jaguars No. 1 wide receiver, drawing rave reviews from new head coach Gus Bradley during the team's mini-camp a few weeks ago.
The Jaguars traded up in the 2012 NFL Draft to draft Blackmon fifth overall and he ended the season with 64 receptions with 865 yards and 5 touchdowns. Blackmon had a run in prior to the 2012 season, being arrested for a DUI a month after the draft.