Second-placed Manchester City's 3-3 home draw with Sunderland on Saturday had given United the chance to ram home their advantage and with seven games remaining they are five points ahead and now look odds-on for glory.
Ecuador winger Valencia, a driving force all game, ended Blackburn's resistance after prolonged United pressure when he came inside and sent a swerving effort into the far corner although he acknowledged it was "50/50" a shot and a cross.
"It was a long night wasn't it? We just had to persevere and persevere and we got our reward for it," United boss Sir Alex Ferguson told Sky Sports.
"We always had them under the cosh and we just had to be careful with the counter-attack."
"Coming so late in the game [the goal] is important. If you don't try to shoot you'll never find out if you are going to score," he said.
Ferguson opted for a midfield three to combat Blackburn's physicality and United were on top from the off.
The second half was a bit more balanced and Blackburn's Morten Gamst Pedersen had the ball in the net but the officials ruled it had gone out of play previously.
Pundits agree that United's side is not as strong as some of their other championship teams and there could still be more twists in the run-in, especially with City hosting United on April 30.
However, Monday's performance was the archetypal title-winning display.
