Valencia, who linked fruitfully with Rafael down the right flank throughout the first period, drilled in a low, right-wing cross which Hernandez redirected against the base of the Rovers post, only for the ball to the back of Paul Robinson, who showed great dexterity to turn and grasp the ball on the line.
United's general confidence in possession suggested a breathrough was inevitable, but Blackburn served notice of their dormant threat in the 18th minute when Junior Hoilett surged from midfield and unleashed a curling, 25-yard effort that De Gea spectacularly punched away.
The hosts' tight, clustered formation meant United's dominance of the ball was given little chance to spawn clear openings. The Reds' first corner, gained just after the half-hour, yielded a chance for Jonny Evans but, after a goal and an assist in his previous two outings, the centre-back could only curl a low shot at Robinson, rather than around him.
Suitably buoyed by that chink in the hosts' armour, the visitors stepped up the pace again. Valencia, enjoying himself as United's most menacing outlet, slung in a superb back-post cross that the unmarked Jones could only direct wide of the upright. Blackburn quickly regrouped, however, and resumed their gameplan of containment.
Just as the interval loomed with United comfortably in charge, Rovers forced two fine saves from De Gea. The Spaniard first produced a wonderfully acrobatic, one-handed stop to turn Marcus Olsson's crisp half-volley over the bar, then flipped Hanley's looping header to safety from the resulting corner.