The 21-year-old ended January with two goals and four assists in the league and was crucial in Tony Mowbray's charges winning four games on the spin.
In addition, Armstrong also scored in the FA Cup third round replay against his former club Newcastle United.
The attacker has twice won the League One version of the award, including while at Ewood Park on loan last February, but this is his first at a permanent club.
He eventually left the Premier League club and joined Blackburn on a permanent contract in the 2018 summer transfer window for a fee reported by Sky Sports to be worth £1.75 million.
Armstrong remains on good terms with Newcastle and told the Blackburn website earlier this year how "the club still means a lot to me".
Meanwhile, Rafael Benitez told Chronicle Live that he chose to let Armstrong go because it was the best decision for him, as he wouldn't be getting much game time at the club - he suggested that the youngster had "no chance of playing too many minutes".
Given how Armstrong is prospering now he's being given first-team football and winning awards, with his young age indicating much more progress to come, is this a decision the Magpies boss will come to regret?
Have something to tell us about this article?