After being frustrated for 81 minutes, the visitors took the lead with a wonder-goal by winger Valencia, whose swerving, rapid effort from an acute angle bamboozled Rovers' defence and goalkeeper to fly into the net.
Just over four minutes later substitute Young – sent on just five minutes earlier – turned on to Valencia’s pass to fire a vicious low drive in off the post, that goal putting Rovers in the drop zone and sparking an early exodus of home support.
Coupled with Manchester City’s 3-3 draw at home to Sunderland, United’s seventh win on the bounce puts them five points clear at the top of the league with seven matches remaining.
Blackburn, meanwhile, slip into the bottom three on goal difference behind Queens Park Rangers, although more feisty performances like this will give them a fighting chance of staying up.
An eerie, subdued atmosphere prevailed in a first half where United dominated but had keeper David De Gea to thank for some acrobatic stops at the end of the 45 minutes.
The Spaniard did very well to first palm away a Marcus Olsson volley and then tip over Grant Hanley’s header from the subsequent corner as Rovers almost snatched the lead in first-half stoppage time.
Otherwise it was all United, who had two thirds of the possession but were denied by some poor finishing from Javier Hernandez of all people, and Wayne Rooney, both of whom mis-hit volleys early on.
The visitors were also denied by a combination of the woodwork and goalkeeper Paul Robinson when, on 11 minutes, Hernandez got his finish right from Valencia’s excellent cross: the fizzing strike rebounded off the near post, back against Robinson and was trickling in before the former England keeper clawed the ball from the line.
De Gea’s late-half saves were not his first of the half as, from a rare Rovers counter, he dived across to keep out Junior Hoilett’s curling shot: Blackburn were indeed more accurate than United with their shooting in the first period, making the Spaniard work more often than Robinson.
But normal business was soon resumed as Jonny Evans fired straight at Robinson after the hosts failed to clear a corner - their second shot on target - while former Blackburn player Phil Jones headed another Valencia cross wide, and Scott Dann’s block from Michael Carrick’s cross looked suspiciously like handball, Howard Webb well placed but pointing for a corner.
The second half saw United continue to apply the pressure, and they went close when a Rooney free-kick was kept out by Robinson, who also denied Rafael a long-range effort when a corner was not fully cleared.
The disappointing Hernandez got his wires crossed when Rovers’ defence let a high ball land for him with just Robinson to beat: possibly thinking he had less time, the Mexico forward mis-timed his header when he could have controlled and shot.
That escape sparked a brief Rovers rally, which saw them unlucky not to take the lead: Ayegbeni Yakubu, hitherto anonymous save for the flick that sent Hoilett in earlier one, fired just over after Carrick gave the ball away; the Nigerian striker then fired just wide after getting outside Evans, the angle too tight for him to test De Gea.
A controversial 62nd-minute free-kick almost saw the hosts go ahead: Paul Scholes was wrongly adjudged to have fouled Yakubu on the edge of the D and Morten Gamst Pedersen’s effort deflected off the wall to leave De Gea only able to parry the ball - fortunately for him, Rio Ferdinand was quickest to react.
Rovers had the ball in the net soon after that but Pedersen’s fine strike was correctly chalked off, Steven N’Zonzi taking it out of play in the build up.
That in itself seemed warning enough to Sir Alex Ferguson, who introduced Ryan Giggs for Phil Jones, the ex-Blackburn defender starting well in midfield but fading in the second half.
Giggs gave United renewed urgency and, most importantly, balance: an odd starting formation that placed Rooney as a left-winger slipped back to a more conventional 4-4-2 as Danny Welbeck partnered his England team-mate up front.
They continued to grind away but Blackburn were resolute and organised, leaving United frustrated until Valencia’s intervention.
He had tried a similar trick earlier – cutting inside and shooting with the outside of his boot, as United’s forwards had previously been a touch sluggish to meet his crosses – but that effort was blocked.
And on 81 minutes the Ecuadorian elected to shoot from a tighter angle, this time generating curl and pace on a ball that looked like a cross but bent wickedly inside to flummox Robinson and nestle in the net.
It was one of those goals that will appear in video compilations for years to come, its quality overshadowing what would ordinarily have been one of the goals of the week when, a few minutes later, Young spun expertly on the edge of the box before firing a savage low effort in off the right-hand post.
Rovers fans had seen enough, leaving their rivals to sing various odes to the other local giants Manchester City, with United edging a step closer to what would be a remarkable Premier League crown, even by their standards.