The full-back was set to move to Ewood Park after being pushed further down the QPR selection order by the arrival of Nedum Onuoha.
Rovers boss Steve Kean has been in the market for a new full-back to boost his side in the battle against Premier League relegation and has succeeded in his search.
"I think the age he's at gives us a little bit more experience in that position," Kean said of Orr ahead of Wednesday's home game against Newcastle United, which will come too soon for the 29-year-old to make his debut.
"We've been playing Jason Lowe and young Adam Henley there as well, but we feel Bradley will bring us experience and another option."
The signing of Orr for Rovers follows the earlier arrival of Marcus Olsson, who joined the Premier League club on a free transfer from Halmstads BK.
Orr signed for QPR in the summer of 2010 but made just six Premier League appearances for the London club
He told Sky Sports News of his latest transfer to Blackburn: "It just really excites me.
"It is obvious they have got a great squad and it is a chance for me to try and establish myself as a Blackburn player and do as well as I can for the club."
Orr is also confident Rovers can avoid relegation, despite currently sitting inside the drop zone after 22 matches.
"It is a fight that I am very optimistic about," he said. "When you look around the squad and you see the quality and experience, as well as youth, it only bodes well."
