Peterborough initially made life difficult as Chris Whelpdale equalised Morten Gamst Pedersen's early opener, but goalkeeper Joe Lewis was sent off just before half-time for clattering Pedersen and Steven Reid slotted home the spot-kick.
Posh managed a second equaliser through George Boyd just after half-time, but Rovers soon made the difference in numbers tell with Michel Salgado, Benni McCarthy and a Nicola Kalinic penalty completing a comprehensive 5-2 victory.
Rovers went ahead when Pedersen rifled a free-kick into the roof of the net, but the Championship side soon levelled when Whelpdale headed home Tommy Rowe's cross.
Lewis was dismissed on the stroke of half-time for bringing down Pedersen in the area, but although Reid confidently converted the penalty, Posh hit back again as Boyd latched onto a pass from Whelpdale before firing a ferocious strike past Jason Brown.
Blackburn made the most of their extra man after the interval and Salgado latched onto a delightful lofted pass from Brett Emerton to grab his first goal for the club before Kalinic set up McCarthy for the fourth and added the fifth himself from the spot after Martin Olsson's cross had been handled.