Blackburn extended their run to one defeat in their last nine matches under new manager Tony Parkes as struggling Portsmouth threw away the lead to crash to yet another defeat.
Portsmouth opened the scoring through veteran striker Steve Claridge in the 14th minute but poor defending from the Fratton Park outfit saw them surrender their advantage in the first half.
Managerless Pompey, who sacked Alan Ball three weeks ago, were in disarray at the back as Christian Dailly smashed in an equaliser in the 33rd minute when the home side failed to clear from a corner from Damien Johnson.
There were signs that Portsmouth were struggling as Blackburn improved throughout the game and just before the interval Norwegian international Egil Ostenstad scored what proved to be the winner.
Ostenstad owed much to the skill of winger Damien Duff, who tormented Portsmouth all afternoon. He beat two defenders before letting fly a fierce cross which former Southampton striker Ostenstad turned into the net at the near post with a header which struck the underside of the crossbar.
Three minutes into the second half, former Aston Villa and Sheffield Wednesday striker Guy Whittingham had a chance to equalise for Portsmouth but his scissors-kick was well saved by Alan Kelly.
Ostenstad continued to torment the Portsmouth defence and saw a left-foot shot fly into the side netting after 54 minutes when he should have done better.
Blackburn lost the services of goalkeeper Kelly in the 62nd minute when he rose at the far post to challenge Darren Moore for a corner and fell awkwardly as he landed.
A crowd of players gathered around Kelly who was in obvious pain and he was carried off on a stretcher to be replaced by Alan Fettis.
Portsmouth had another chance to get back into the game soon afterwards but Claridge had strayed into an offside position when he received a pass from Sammy Igoe and then blazed his effort against the post.