Norwegian Morten Gamst Pedersen scored a scorching equalizer, his third goal in two games against United this season, five minutes after Louis Saha's crisp finish gave the visitor the lead on the half hour.
Tuesday's first semifinal may have been oddly subdued but this was the polar opposite, fizzing with passion almost from the off. Yet even if United's desire - unlike Arsenal's the previous night at Wigan - could hardly be questioned, the Red Devils began dreadfully.
A misplaced Rio Ferdinand backpass in the opening seconds suggested the depth of their unease following the draw with non-league Burton on Sunday, and with more composure Blackburn could have been two up inside ten minutes.
First Robbie Savage, arriving unmarked in the center of the box, volleyed straight at Edwin van der Sar after being picked out by Craig Bellamy.
Then the Welsh forward was unfortunate that Pedersen's cross deflected behind him after he too was left unattended.
But it was United, against the run of play, which took the lead.
Sir Alex Ferguson, despite his protestations that he would field a weakened side, had left out only Ruud van Nistelrooy of a putative first eleven, and it was the man who replaced him, Saha, who scored the goal.
The Frenchman shot home confidently from the edge of the box after being played in by Ryan Giggs.
That was harsh on Rovers but they were level within six minutes. Shefki Kuqi, energized after an incident in which he was penalized for a foul on van der Sar when the culprit appeared to be Mikael Silvestre, flicked on Ryan Nelsen's long pass and Pedersen darted in from the left to thrash a magnificent finish into the top corner.
Bellamy had been booked for his protests after the earlier decision against Kuqi - a caution that will keep him out of Blackburn's return to his former club Newcastle - and the frustration had evidently not quite dissipated when the referee Rob Styles inadvertently blocked a Blackburn clearance eight minutes before halftime.
Wayne Rooney gathered the loose ball and was fouled by Savage, who was promptly scythed down by Alan Smith.
In the subsequent melee, Rooney and Savage engaged in some low-key grappling and all three were booked.
Rooney is now suspended for next week's FA Cup replay against Burton but that was not the end of the cards as five further bookings followed in the second half.