Rafael hit the base of the post less than 60 seconds after the restart with a smart volley but it was as close as the Reds came to drawing level before Blackburn struck again just on the half-hour. Rochina nabbed his second with a strong finish into the roof of the net from 12 yards after a smart backheel from Michael Potts had played in the Spaniard.
Neither side fashioned a further chance in the first period and the Reds made a double change at the start of the second. Rafael made way for Davide Petrucci, while Nicky Ajose replaced Oliver Norwood.
That prompted a switch in formation, too, as United went from 4-5-1 to a loose 4-3-3, with Ajose flanked by King and Obertan. It almost paid immediate dividends when Bunn spilled a stinging Petrucci shot before doing well to block Ajose's close-range rebound.
The game then threatened to boil over when Robbie Brady and Reece Brown were booked in quick succession for full-blooded challenges (Brown's ended Rochina's involvement in the game), but referee Dean Mohareb did well to diffuse the situation.
United continued to probe for a way back into the game and were duly rewarded on 65 minutes when King's driving run was halted inside the penalty area. Substitue Petrucci stepped up to tuck away the spot-kick and reduce the arrears.
Blackburn's two-goal lead would have been restored a minute later had Ben Amos not stood firm as Rovers substitute Tom Hitchock bore down on him. The Reds goalkeeper was then relieved to see Mauro Formica miss the target after Amos' heavy touch gifted the midfielder a sight of goal.
With a place in the national play-off at stake, United threw men forward in search of a second goal. Petrucci hit the target from distance and Obertan rippled the side-netting, but Blackburn stood firm to see off the Reds' late rally and book a date with Chelsea at Stamford Bridge next week.
Remember, though: United's league challenge may be over but there's still a Lancashire Senior Cup