Two goals from Benjani heaped more pressure on under-fire Liverpool boss Roy Hodgson as his side were comprehensively beaten at Ewood Park.
Blackburn struck first, Martin Olsson coolly driving through Pepe Reina's legs before Benjani smashed a second.
And Rovers drove on against hapless Liverpool, Benjani tapping in a close-range third after David Hoilett's run.
Steven Gerrard managed a consolation goal but then blazed a penalty over the bar with six minutes to go.
Liverpool's failure to string a run of results together this season has put Hodgson, whose side have only won once on the road, under significant pressure.
The Englishman came under heavy fire after losing to Wolves over Christmas but their subsequent 2-1 win over Bolton appeared to have bought him some time.
However, this will rank amongst one of Liverpool's worst setbacks this season, along with the 3-1 loss against Newcastle, and 2-0 defeats by Stoke and Everton, and it is difficult to see how he will be able to avoid more negative headlines after a disastrously below-par display which will leave the doom mongers sharpening their knives.
It could have been so different, had Liverpool built on a promising opening five minutes during which Fernando Torres headed wide after a clever one-two with Paul Konchesky.
But inconsistency has dogged both these mid-table sides this season, with each starting the game on 25 points. And Liverpool showed their incredibly flaky nature by losing their momentum almost immediately.
Blackburn boss Steve Kean has also been under pressure after replacing Sam Allardyce, who was fired by Venky's, the club's new owners.
But despite that sacking and a seemingly failed bid for Brazilian star Ronaldinho this week, there is an air of optimism beginning to circle around Ewood Park, with talk of big money being made available by their Indian owners.
And the players fed off that, showing constant endeavour and creating a series of chances on the counter attack.
Mame Diouf had the best of these when Olsson's wicked cross flashed beyond Martin Skrtel but it took the striker by surprise and he could only shin the ball into the hands of Pepe Reina.
Even with Gerrard, Torres and Joe Cole in the starting line-up, Liverpool attacks were in short supply and Rovers took a deserved lead on the half-hour mark.
Ryan Nelsen's long ball found Diouf who played in the overlapping Olsson and the Swede drove his finish through the legs of Reina.
And Blackburn heaped more misery on Liverpool before half-time when Benjani picked up the ball inside the box, spun Sotirios Kyrgiakos expertly and smashed a fierce effort past Reina and into the roof of the net.
Hodgson looked on crestfallen from the sidelines, with his hands planted in his coat pockets and that was a mood echoed by his players' body language on the pitch.
Almost inevitably, Benjani, who had not started since 6 November and scored once in his previous 12 appearances, made it 3-0 on 56 minutes.
Hoilett skinned Martin Skrtel as he hit the bye-line and broke into the box before squaring to Benjani who tapped into the net easily to leave the crowd cruelly singing: "you're getting sacked in the morning" in the direction of Hodgson.
Liverpool did manage to dominate possession in the second half but that was primarily a product of Blackburn sitting back on their healthy lead and relying on the superb centre-back partnership of Nelsen and Christopher Samba, who Rovers will be desperately keen to keep at Ewood Park.
Joe Cole's 25-yard drive was well saved by Mark Bunn as Liverpool finally fired a warning shot.
And they did pull one back when Gerrard, playing his 550th game for Liverpool, lashed into the corner from close range.
Gerrard was then handed a chance to make the remaining six minutes extremely tense when he was brought down by Michel Salgado in the box.
But the Liverpool captain blazed his penalty well over the bar to compound the Reds' agony.