John Buckley hit a late equaliser as Rovers earned a point despite not being at their best against a Stoke City side that finished the game with ten men.
Buckley struck from close range late on just three minutes after coming off the bench, with his leveller arriving after Nick Powell's first half opener.
Rovers played the final ten minutes against a City side who had James Chester sent off for hauling down Armstrong when the diminutive frontman went clean through.
In the end, Rovers will be relieved to have taken a point, with Stoke hitting the post and also seeing Thomas Kaminski in fine form throughout the contest.
The standout save was when the Belgian clawed away another Powell header just moments after the playmaker put the visitors in front.
Tony Mowbray made four changes from the last league outing against Birmingham City, with Ryan Nyambe, Jarrad Branthwaite, Joe Rothwell and Bradley Dack all starting.
It was a debut for on loan Everton defender Branthwaite, whilst Dack's first start since returning from injury was also his 100th in the league for Rovers since arriving from Gillingham in the summer of 2017.
It was a half of few chances at a chilly Ewood Park, with Armstrong seeing his strike wrongly chalked off after being adjudged offside midway through the first half.
However, with their first real chance of the half, Stoke took the lead and it came from the man who scored the winner in the meeting at the bet365 Stadium last month.
Jordan Thompson's inswinging corner was dangerous and Powell was there to head home from inside the six-yard box with only seven minutes of the half remaining.
It could have been worse for Rovers going into the break, with Thomas Kaminski getting the slightest of touches on Thompson's strike to divert it onto the post before Powell smashed wide.
Kaminski again was called into action a moment later to pull off a sensational save to stop Powell from scoring another header as the visitors finished the half the strongest.
Rovers would have counted themselves fortunate to have gone into the interval just a goal down, but needed a much-improved display in the second 45 minutes.
A double change at the break saw Bradley Johnson and Rothwell replaced by Jacob Davenport and Sam Gallagher, but the latter's afternoon lasted just 15 minutes as injury forced him off.
Both sides were struggling to create much, but substitute Lewis Holtby's hooked cross was hacked clear before Dack could get on the end of it from close range.
That was to be Dack's last involvement as he was replaced by Buckley, and Rovers worked their best opportunity of the game soon after.
A deep cross from Harvey Elliott was flicked on by Brereton, and after Nyambe's pull back was met well by Armstrong but Josef Bursik was there to beat behind.
However, there was nothing Bursik could do as Rovers opened the door and forced home an equaliser with just 15 minutes remaining.
A ball across from right to left was just about kept alive by Elliott, and Buckley was there to tap home just three minutes after entering the action.
More drama arrived a few moments later as visiting skipper Chester was shown a straight red card for tripping Armstrong when the frontman went towards goal with just the goalkeeper to beat.
Although there was a late rally from Rovers, most notably when Brereton's dangerous cross just evaded the impressive Davenport, both sides took a share of the spoils.
Next up for Mowbray's men is a home test against Swansea City on Tuesday 19th January, kick-off 7pm.
Rovers: Kaminski, Nyambe, Lenihan (c), Branthwaite, Douglas, Johnson (Davenport, 46), Travis (Holtby, 64), Rothwell (Gallagher, 46) (Brereton, 61), Dack (Buckley, 72), Elliott, Armstrong.
Unused substitutes: Pears, Downing, Trybull, Dolan.
Goal: Buckley, 76.
Booked: Travis, 36
Stoke City: Bursik, Collins, Souttar, Chester (c), Brown, Allen (Cousins, 83), Obi, Thompson, Norrington-Davies, Powell (Clarke, 61), Matondo (Baath, 83).
Unused substitutes: Nna Noukeu, Ince, Vokes, Smith, Oakley-Boothe, Clucas.
Goal: Powell, 38
Booked: Thompson, 48, Norrington-Davies, 71.
Sent off: Chester, 79.
Referee: Geoff Eltringham.
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