Blackmon, 30, was out indefinitely after he suffered a broken finger last week. He signed a two-year, $1.875 million contract with the Jaguars back in March.
Blackmon is owed a base salary of $870,000 for the 2015 season and has a cap figure of $945,000, according to Spotrac.
The Bengals claimed Fortt off of waivers from the Saints not too long ago. New Orleans placed Fortt on injured reserve with the designation to return a few days before the start of the regular season, but later waived him after reportedly he missed a few meetings.
Fortt, 22, played two years for Penn State before transferring to California last year. In 2013, he started all nine games for California totaled 64 tackles, 0.5 sack and 3.5 tackles for a loss.