United started well, but slid from mediocre to bad to worse. Despite some indifferent defending and disjointed midfield play, United did at least create a hatful of chances in an exciting first half.
But United's second-half display descended into the depths of despair as they hardly troubled Rovers' solid defence.
Arsenal's 4-1 home win over Norwich saw them leapfrog over United into second place on goal difference and the prospect of the Red Devils having to face a tricky Champions League qualifier before the start of next season now looms large.
Blackburn created few chances, but only the most die-hard United fan would have denied Rovers manager Mark Hughes - the former Old Trafford hero - his team's point.
The game started in the kind of style which suggested thrills, spills and goals. After only 50 seconds, United defender Rio Ferdinand failed to cut out a long ball out of the Blackburn defence in his own penalty area.
The ball fell for Jonathan Stead, who shot straight at United goalkeeper Tim Howard from 12 yards out. Twenty seconds later, United could have been ahead.
Wayne Rooney's cross from the left side of the Rovers box came back to the England youngster and his ten-yard header was pushed away by the diving Blackburn goalkeeper Brad Friedel.
United lost Ryan Giggs with a hamstring injury in the sixth minute and Roy Keane came on as his replacement.
Blackburn's David Thompson and Cristiano Ronaldo of United both saw 20-yard shots fly over the bar and Rovers missed their best chance of the game in the 13th minute.
Steven Reid's blistering 25-yard drive took a wicked deflection off Quinton Fortune, forcing Howard to hurl himself across goal to execute a fine save. The ball ran to Andy Todd, only six yards out, but with Howard still grounded the Blackburn skipper ballooned his close-range shot over the bar.
It was Freidel's turn to save his side six minutes later. Paul Scholes fired over his free-kick from the right corner of the Blackburn box to the far post where Mikael Silvestre connected to produce a fine downward header which was brilliantly saved by Friedel.
A piece of Rooney magic almost broke the deadlock in the 25th minute.
Keane won the ball in a tackle with Eire international team-mate Reid in the centre circle and turned it on to Ruud van Nistelrooy. The Dutch striker found Rooney, who hit a screaming 30-yard right-foot shot that crashed against the post with Friedel beaten.
Seconds later, Ronaldo was narrowly wide with a snap 25-yard shot.
At the other end, Howard did well to punch away a fierce free-kick from Morten Gamst Pedersen on the right but it was United who were doing most of the pressing.
Ronaldo's cross from the right could have gone anywhere but it hit Aaron Mokoena in the middle of the box - and went wide.
The woodwork again came to Blackburn's rescue in the 38th minute. Gary Neville, John O'Shea and van Nistelrooy carved an opening for Ronaldo, and Blackburn failed to clear his cross from the right.
Keane, with a stamina defying his 34 years, raced onto the ball to hit a low shot from the edge of the box which hit a post.
Two minutes later, Scholes produced another fine cross from the right and Silvestre was again on hand at the far post to get in a powerful header which was cleared off the line by the diving Pedersen.
Pedersen came within a whisker of scoring at the other end when his glancing header from Reid's left-wing cross flashed narrowly wide of the goal.
United twice came close to scoring in the dying minutes of the half. O'Shea set up Ronaldo on the right side of the Blackburn box and his jinking run ended with a shot which was well saved by Friedel's legs.
Then, O'Shea flicked on a Rooney corner for Scholes to drive narrowly wide at the near post. As United's fans waited for the goal to come after the break their hero's simply ran out of ideas.
Alan Smith replaced the ineffective van Nistelrooy in the 63rd minute, but the momentum had by then dropped drastically.
O'Shea fired a 25-yard drive straight at Friedel four minutes later, while Rooney and Smith both had half-chances.
But the only time Freidel came close to breaking sweat in the second period came in the 85th minute when he was forced to dive and save a mis-hit shot by Gary Neville.
Blackburn had only one real second-half chance - Stead clipping a 12-yard shot narrowly wide on the hour.
MAN OF THE MATCH: Brad Friedel (Blackburn) - The American keeper made three crucial saves to salvage a point for Rovers at Old Trafford. He was also aided by the post as his team earned a deserved draw.