They also elevated linebacker Micah Baskerville and tight end Stephen Carlson from the practice squad to the active roster for the game.
St. Brown and Blackwell both practiced fully Wednesday and were listed as questionable.
St. Brown missed the past four games after injuring a hamstring in an Oct. 5 game against the Commanders. The Bears made him a healthy scratch the first three weeks of the season.
He has appeared in two games with one catch for 21 yards.
Blackwell has played only two games because of a hamstring injury, recording two tackles in Week 1 against the Packers and two tackles in Week 3 against the Chiefs.
Running back Khalil Herbert also designated to return from injured reserve, returning to practice this week, with a questionable designation after practicing in full Wednesday. But Herbert remains on IR and will not play tonight.
Baskerville was flexed from the practice squad for the second consecutive week. The undrafted rookie from LSU made his NFL debut last Sunday against the Saints, playing 10 snaps on special teams.
Carlson has not played in an NFL game since 2020 when he appeared in all 16 contests with two starts for the Browns, catching one pass for 11 yards.