Jason Roberts climbed off the bench to hit a late double as Blackburn came from behind to claim three priceless Barclays Premier League points with a 3-1 win over fellow strugglers Portsmouth. [LNB] Jamie O'Hara had put Pompey in front with a 30-yard effort in the 15th minute, but things changed after Sam Allardyce rang the changes at the break. [LNB] Roberts steered home a 52nd-minute equaliser, before Ryan Nelsen headed Rovers in front with a rare strike in the 73rd minute. Roberts then wrapped up the points with five minutes to go.[LNB] Rovers started best in their bid to exit the drop zone, a Pascal Chimbonda shot causing chaos in the Pompey box and almost presenting a chance for El-Hadji Diouf. [LNB] Younes Kaboul and O'Hara's low shots dribbled wide for the visitors while Anthony Vanden Borre messed up a half-chance after being fed in the box by Kevin Prince-Boateng. [LNB] Pompey's lead came out of nowhere when Aruna Dindane fed O'Hara and he found a way past Paul Robinson with a low shot from 30 yards. [LNB] Rovers' woes were summed up in the 33rd minute when, having won a free-kick 30 yards from goal, the recalled Morten Gamst Pedersen lofted a pitiful set-piece attempt high and wide. [LNB] After boos greeted the end of a dismal first period, manager Allardyce had no choice but to make changes, introducing Roberts and Benni McCarthy in place of Diouf and Pedersen. But Pompey came close to grabbing a second in the 50th minute when O'Hara curled in a free-kick from the left and Kaboul rose to glance his header just wide. [LNB] Allardyce's double change paid dividends when Nelsen headed down David Dunn's corner in the box and Roberts reacted quickest to steer the equaliser past David James. [LNB] Nelsen's superb 73rd-minute header put the home side in front and Roberts wrapped up the points five minutes from time.