With seven rounds left, United is clear of crosstown rival Manchester City and favorite to win its 20th league title — although manager Alex Ferguson said he is taking nothing for granted.
"In my experience of these situations, it doesn't matter what the points are at the moment," Ferguson said.
Valencia cut in from the right wing and fired a swerving shot from a tight angle that flew past goalkeeper Paul Robinson and into the far corner in the 81st minute. Young made sure of the win with a low shot in the 86th.
"It was a long night. We had to persevere and persevere," Ferguson said. "We weren't making any clear-cut chances for a long time. But it was an important win and it typifies the history of the club."
Blackburn, which dropped into the relegation places on goal difference, had a goal disallowed when Morten Gamst Pedersen's 63rd-minute shot was ruled out because the ball went out of play in the buildup.
United kept the ball for long periods in the match at Ewood Park, but Blackburn's defense kept things tight at the back to threaten a frustrating evening for Alex Ferguson's title-chasing team.
Wayne Rooney, who had been dropped for United's surprise 3-2 home loss to Blackburn in December, created the game's first chance in the 10th minute. He hit a long pass that Blackburn defender Grant Hanley struggled to clear, but Javier Hernandez dragged his shot wide under pressure from left back Martin Olsson.
The Mexican striker went close again soon after, hitting the post from Antonio Valencia's low cross.
Blackburn manager Steve Kean had opted for a five-man midfield to try and hit United on the counterattack, and the tactic almost paid off when Junior Hoilett forced goalkeeper David De Gea into a diving save after receiving Ayegbeni Yakubu's clever pass in the 21st.
United had glimpses of goal when Jonny Evans' shot was saved by Robinson following a 34th-minute corner, and Phil Jones headed over the bar from Valencia's cross in the 38th.
Against the run of play, Blackburn almost took the lead just before halftime when De Gea failed to punch a cross clear. Marcus Olsson volleyed the loose ball back at goal from the edge of the box, but De Gea redeemed himself with a flying save.
The Spanish goalkeeper, who endured a tormented time in the December defeat, produced another fine stop to tip Hanley's header from the ensuing corner over the bar and later earn Ferguson's praise.
"The progress is obvious for all to see," Ferguson said. "He's growing in stature and confidence. He made three great first-half saves. David kept us in the game."
Blackburn was still proving potent on the break, however, and Morten Gamst Pedersen fired a low free kick that De Gea smothered in the 62nd.
The home fans thought Blackburn had taken the lead soon after when Pedersen lashed the ball into the net, but the ball was rightly ruled to have crossed the line before Steven Nzonzi's cross came over.
"It could have gone either way," Blackburn manager Steve Kean said. "But you've got to take your chances. I thought we created lots, albeit on the break."
With Blackburn sticking to its game plan, Rio Ferdinand needed to make a last-ditch challenge to prevent Yakubu converting a low cross in the 77th.
United finally broke the deadlock when Valencia rifled a shot in from the right that flew past Robinson and into the far corner.
When asked whether the effort was meant as a cross or a shot, Valencia admitted it was "50-50."
That was no consolation to the Blackburn manager, however.
"We are just a little bit frustrated, because we felt if we could have held on for another five minutes we could have really frustrated them," Kean said.
As the massed United fans sang in celebration, Young, on as a second-half substitute, finished Blackburn off with a low curling shot.