It looked like being a frustrating night for United, who dominated for long periods in the Premier League clash at Ewood Park but failed to get closer than an early Javier Hernandez shot that struck a post.
Indeed, David de Gea made a string of fine saves to repel Blackburn counter-attacks before Valencia and Young struck near the end.
Before the game, Manchester City boss Roberto Mancini had expressed his hope that Rovers would continue their sequence of just two defeats in 12 home league games against their north-west neighbours to breathe new life into the title race.
If he was watching, the Italian could have no complaints about Blackburn's performance.
But after a succession of excellent saves from De Gea, United finally made their own pressure tell, with Valencia breaking the deadlock 10 minutes from time before Young added another.
The result also saw Rovers drop back into the bottom three on goal difference.
That is of little interest in Manchester, though.
City's failure to beat Sunderland on Saturday had created a fevered atmosphere amongst the 7,000-strong visiting contingent.
Having celebrated winning title number 19 in the corresponding fixture last term, they were ready to party again, believing a 20th was on the way.
And for 20 minutes, it seemed the United players were ready to sweep Blackburn away on the tidal wave of support.
Valencia was an ever-present threat on the right flank and, whilst Wayne Rooney was operating from a position both wider and deeper than his most threatening, Javier Hernandez's movement caused Blackburn further concern.
The Mexican had already come close once when he nipped in at the near post to meet another low Valencia cross.
From less than six yards, it seemed a certain goal.
But somehow, Hernandez's effort came back off the base of a post and before the rebound bounced in, Paul Robinson had turned round and grabbed it.
As United poured forward and Paul Scholes and Michael Carrick found plenty of space in front of the Rovers' back four, it seemed just a matter of time before the visitors opened their account.
Carrick curled a shot over and had a penalty claim turned down.
Against his former club, Phil Jones misdirected a header wide, as did Patrice Evra.
Blackburn took heart from this unexpected reprieve and, buoyed by a combination of their excellent record in this fixture and two wins from their last three games, advanced themselves.
It was after a particularly dire performance in the Old Trafford encounter between these sides on New Year's Eve that De Gea was axed.
Such has been the dramatic improvement since then, though, that it was not much of a surprise when the Spain Under-21 star tipped a Junior Hoilett effort over.
Then, in a frantic final minute before the break, both Marcus Olsson and Grant Hanley - Blackburn's Old Trafford match-winner - were repelled.
It marked a change in attitude from the hosts, who continued to push forward with gusto when the teams re-emerged.
Scott Dann glanced a header wide, top scorer Yakubu wasted a couple of decent opportunities and Rio Ferdinand had to rescue De Gea who, after all that had gone before, blotted his copybook by spilling Morten Gamst Pedersen's thunderous effort.
After shock wins for Wigan and QPR against the Premier League's so-called giants over the past fortnight, Blackburn thought they had put themselves in line for another when Pedersen rasped home from 20 yards.
The home supporters were deflated, though, as it was ruled the ball had just crossed the line for a goal-kick before Steven Nzonzi pulled it back.
More heroics from Robinson to cut out a low Rooney cross in the middle of a crowd of bodies set Blackburn on their way again.
Had Ferdinand not managed to get to Yakubu in the nick of time, the Nigerian would surely have touched home Marcus Olsson's low cross at the near post.
United were starting to get desperate. But not for long.
Valencia has missed so much of this season through injury but there is a strong argument that he is the man of their campaign.
With time running out, it was the Ecuador wide-man who struck, drilling a shot past Robinson after darting into the box.
Young added another before the end with a neat finish from the edge of the box which just rubbed it in.
United's next assignment is at home to QPR on Sunday, before City take on Arsenal at the Emirates. Blackburn travel to West Brom on Saturday.
