The Bears also downgraded defensive back Tyrique Stevenson to questionable for the game, according to a press release.
Finally, the Bears elevated linebacker Micah Baskerville and tight end Stephen Carlson from the practice squad for the game.
St. Brown had spent the last four weeks on injured reserve after suffering a hamstring injury. He has only appeared in two games this season, with one catch for 21 yards.
Blackwell was also dealing with a hamstring injury, and had appeared in two games this season, with four tackles in those contests.
The Bears also had the option to activate running back Khalil Herbert, but he had been listed as questionable for the game. He will not play on Thursday, but could return for the team’s next game.
Stevenson had been dealing with an ankle injury during the week, and may be limited if he does indeed suit up for the contest.