Rangers' hopes suffered a blow when Rowlands was shown a straight red card for a two-footed tackle on Greening.
Despite the dismissal, the hosts started the second half well and Blackstock shot just over the bar from 20 yards.
But the game turned Albion's way eight minutes after the break when half-time substitute Kim headed in Gera's cross from close range to give the visitors the lead.
Brunt then hit the bar from the edge of the area but he was not to be denied and he sealed the points, and all that went with them, by hammering a free-kick into the top corner from just outside the area.
The victory won Albion their first trophy for 40 years as they finished two points clear of Stoke.
QPR coach Luigi De Canio: "We prepared ourselves for a great game so that we could celebrate the end of our season but I believe the referee ruined the whole game.
"He offended me, he offended the fans and he offended my players.
"I didn't think it was a red card but I'd like to say well done to West Brom, who are worthy champions."
West Brom boss Tony Mowbray: "This is a proud moment. When you think back to when we lost to Burnley on the opening day, we've come a long way and made a lot of progress.
"Credit to the players who have won more games and scored more goals than anyone else and rightly ended up as champions.
"Hopefully we will have a good shot at it at the next level."
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