Diouf had two goals ruled out by a linesman's flag
A Keith Andrews goal earned Blackburn a deserved share of the spoils against a resolute West Ham after Mark Noble had given the visitors the lead.
Noble's superb, curling finish with the outside of his foot gave West Ham the lead against the run of play.
Andrews equalised for Blackburn, following up a half-cleared Morten Gamst Pedersen throw to rifle in.
El-Hadji Diouf twice had goals ruled out by a linesman's flag as Blackburn largely dominated the game.
Blackburn manager Sam Allardyce had intimated before the match that West Ham's league position, nine places above Rovers in seventh, was as a result of their defensive resolve, a virtue he largely attributed to their assistant manager Steve Clarke.
If this is indeed the case Clarke can feel rightly proud of his role in the earning of this hard-earned point for the Hammers.
Blackburn pressed throughout and central to most of the threat they posed was towering centre-back Christopher Samba, primarily from the delivery of Pedersen.
We deserved better - Allardyce
After 10 minutes his header from a Pedersen throw fell into the path of Benni McCarthy, but the striker showed all the rustiness of a man who had been out for a month in miscuing his shot well over from 10 yards.
Two minutes later Samba's sheer presence in the West Ham penalty area for a corner resulted in a melee with the ball ricocheting around before it fell to Pedersen, but his shot was well blocked by the diving Jonathan Spector.
In one instance Samba almost proved as destructive in his own half.
West Ham midfielder Noble, crossed from the right and the Congolese defender rose with goalkeeper Paul Robinson in a confused attempt by both to clear only for the ball to fall to Diego Tristan. Fortunately for the home side, the striker could not get his shot away and the ball was cleared.
At the other end West Ham's defensive resolve was coming under increasing pressure but their response was a testimony to Allardyce's pre-match assessment of their virtues.
On numerous occasions Hammers defenders threw themselves in the way of Rovers players to block goalbound strikes, so much so that goalkeeper Robert Green barely touched the ball.
On the half hour Blackburn did have the ball in the net as Samba's presence again created room, this time for Diouf, who turned and poked in.
However, the linesman's flag ruled it out with replays illustrating that the Senegalese striker was both offside and then infringed against Green.
As a reward more for their defensive application than their adventurous spirit West Ham took the lead just before half-time.
Noble ran 30 yards and fed Boa Morte, who replays later revealed to be offside, and the Portuguese in turn found Tristan in the box. The Spaniard's subsequent lay-off was curled expertly into the far bottom corner by the outside of Noble's right foot.
It took only five minutes of the second half for Blackburn to draw level.
Zola pleased with hard-fought draw
Pedersen's arrowed long throw from the left was only half cleared by Herita Ilunga and Andrews was on hand to rifle in to the far right corner of the net from an angle.
With their tails up Blackburn seized the attacking initiative again but were denied once again by a linesman's flag when a diving Green could only palm out Samba's shot to Diouf who side-footed in but from an offside position.
Blackburn pushed hard for a winner but time and again they were denied by West Ham's resolute and dedicated backline.
In truth West Ham may feel they could have grabbed all three points when substitute Kieron Dyer raced through only to be felled by the backtracking Samba.
Dyer animatedly remonstrated with referee Chris Foy, intimating that his shirt had been pulled, but the official waved his pleas away.
Blackburn manager Sam Allardyce:
"We feel bitterly disappointed we have not got three points and in many ways that is our fault but it is also the fault of the referee's assistant and it is a hard one to take.
"We had two goals disallowed for offside but unfortunately the key area lies with the West Ham goal which nobody can argue is not offside - and we don't know why the referee has not given it (as he was) in such a good position.
"What can be done about it? Nothing. But I have to express my disapproval publicly because we are in a very difficult position and we can't afford to have major decisions going against us."
West Ham manager Gianfranco Zola:
"I think there might have been a penalty for us as well when Kieron Dyer was fouled so that would balance it out.
"When you have got a big team like they are who boom balls into the box it is not easy for anybody.
"My team were excellent really and it is a pity we did not have the extra strength that might have allowed us to cause them a few more problems. But it was a close game and both teams could have won it."
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