Goals by Ruud van Nistelrooy and Louis Saha booked United their Cup Final place at the Millennium Stadium in Cardiff next month.
Steven Reid had brought Blackburn back into the game when he equalised van Nistelrooy's opener, but Rovers created few chances and a measure of United's dominance came with the fact that goalkeeper Brad Friedel was Rovers' best player.
Friedel was magnificent as he saved van Nistelrooy's first-half penalty and then twice denied the Dutchman with superb reflex stops.
After drawing the first leg 1-1 at Ewood Park a fortnight ago, it was United that drew first blood when they opened the scoring in the eighth minute.
Ryan Giggs threaded the ball through to United's man of the match Wayne Rooney and he set off on one of his characteristic trademark runs.
He had an element of luck with a double ricochet off Michael Gray before he touched the ball on to van Nistelrooy who scored with a low right-foot shot from 12 yards with Friedel getting a touch before the shot nestled in the corner of the net.
Giggs was forced off with a hamstring injury five minutes later and Blackburn came back to dominate much of the rest of the half. Fine clearances from Wes Brown and Patrice Evra denied Shefqi Kuqi and Reid but the latter was not to be thwarted in the 32nd minute.
Kieran Richardson made a complete hash of clearing a cross from the left and he eventually kicked the ball against Brown and the rebound fell to Reid, who rifled his right-foot shot home from 12 yards on the right side of the United box.
Gary Neville's poor back-pass almost let in Morten Gamst Pedersen, but the United defender was rescued by Edwin van der Sar's brave clearance.
Fortune favoured United in the 42nd minute when van Nistelrooy pressurised Zurab Khisanishvili and an unfortunate bobble saw the Rovers player handle the ball in the Blackburn box.
Referee Graham Poll awarded the penalty but Friedel guessed right when he dived to his right to tip away van Nistelrooy's right-foot spot-kick.
Friedel produced another magnificent stop three minutes after the break when he hurled himself across goal to tip away van Nistelrooy's header.
But Friedel was beaten two minutes later when United regained the lead. Darren Fletcher robbed Tugay inside the Blackburn half and turned the ball onto Rooney. The England man's delicate chip across the box from the right set up Saha to score with a glorious left-foot shot.
Blackburn had claims for a penalty turned away when Kuqi's shot appeared to hit Rio Ferdinand's hand in the box but Friedel made two more fine saves to deny Rooney and van Nistelrooy as United held on to victory.