After a goalless first half, Darren Bent fired the visitors in front following a goalmouth scramble. Morten Gamst Pedersen provided an immediate response, before Bent grabbed his second of the game with a smart finish.
Diouf then came off the bench to head home the equaliser and Rovers ought to have won it late on when Nikola Kalinic headed wide and Chris Samba missed an open goal.
Rovers have now gone eight league games without a win, while Sunderland haven't won on their travels since the opening day of the season.
Before the game, both managers made changes on the back of 1-1 Boxing Day draws.
For Blackburn, Jason Roberts was handed a rare starting role, with Franco di Santo dropping to the bench, while Pedersen was preferred to Michel Salgado on the wing.
Steve Bruce had to cope with a mini-injury crisis, with John Mensah, Steed Malbranque, Andy Reid and Kenwyne Jones adding their names to a lengthy list that already included Craig Gordon, Anton Ferdinand, Lee Cattermole and Kieran Richardson.
Fraizer Campbell started up front with hot shot Bent, while midfielder David Meyler was handed his debut for the club.
Between them, the two teams had gone 13 games without a league win and you could understand why given the wayward shooting early on.
Just 45 seconds had elapsed when Jordan Henderson played a one-two with Bent, before dragging his effort well wide of the target.
Keith Andrews then fizzed a first-time volley just past the post for the home side, before Benni McCarthy sent a looping header onto the roof of the net from Pedersen's deep cross.
At the midway point of the half, Campbell and then Henderson both tried their luck from long range, but both volleys failed to trouble Paul Robinson in the Rovers goal.
Following his range-finder earlier on, Andrews again let fly from just outside the box, but his superb strike whistled inches wide of the upright.
After a fairly forgettable first half, the game sprung to life seven minutes after the restart. Daryl Murphy made an excellent break down the left, his centre was parried by Robinson and, with Ryan Nelsen and Gael Givet both frantically trying to clear the ball while on the floor, Bent stepped in to smash home his 12th league goal of the campaign.
However, straight from the restart and Rovers were back level. Givet made an interception in midfield, McCarthy touched the ball out left to Pedersen, and he cut inside Nyron Nosworthy before firing a right-foot shoot in off the near post.
Pedersen then almost turned provider when his perfectly-weighted cross was met by Andrews at the far post, but the Irishman somehow side-footed wide.
And Rovers were made to pay, as Sunderland struck on the break. Henderson played a defence-splitting pass for Bent to run onto and the England international coolly stroked the ball past Robinson into the bottom corner of the net.
Sam Allardyce went for broke with a triple change, bringing on di Santo, Kalinic and Diouf. And it proved to be a managerial masterstroke as, just three minutes later, two former Sunderland players combined, as Pascal Chimbonda crossed and Diouf rose highest to head home.
Rovers had the momentum and ought to have won it late on. On 87 minutes, Kalinic's header from Pedersen's cross dropped agonisingly wide of the far post and, two minutes later, Samba missed an open goal after a superb save from Fulop.