It meant John Gregory's side go four points clear of the relegation zone with six games remaining.
Preston's hopes of automatic promotion this season however took a massive blow with a defeat which leaves them six points adrift of the top two.
The first half was all QPR's as Gareth Ainsworth headed narrowly wide from a Lee Cook corner on nine minutes, while striker Marc Nygaard missed a sitter from just five yards out on 36 minutes.
Preston legend Ainsworth also got clear five minutes from half-time but chose to pull the ball back to nobody rather than shoot like he should have done.
The only other action for the visitors before the break was a Matt Hill header on 27 minutes which was palmed away by England Under-21 keeper Lee Camp.
It was going to take something a bit special to break the deadlock and striker Blackstock certainly came up with it five minutes after the interval.
The former Southampton man took a long ball down on his chest 25 yards out and then turned to hit a stunning volley which gave Preston keeper Andy Lonergan no chance.
Preston did fire a warning shot six minutes later when Michael Ricketts hit a free-kick inches wide.
And with ten minutes remaining, Rangers old boy Daniele Dichio forced a fine save out of Camp.
But for all their effort, Preston could not find a goal and the Londoners held on for a famous three points.