Goals from Ayegbeni Yakubu either side of the interval put the Barclays Premier League's bottom club into a surprise two-goal lead before United hit back through Dimitar Berbatov.
There seemed to be only one conclusion after the Bulgarian had scored twice to take his tally to five over the Christmas period.
But despite piling on the pressure, it was United who were undone as David de Gea failed to collect Morten Gamst Pedersen's corner, allowing Grant Hanley to head home the winner 10 minutes from time.
It represented Steve Kean's best win as Blackburn manager, taking them off the foot of the table, and left Ferguson to question the wisdom of not even putting a rested Wayne Rooney on the bench on the day he confirmed an intention to remain in his job for a further three years.
The party atmosphere at kick-off proved to be a false guide to what followed.
In a pre-recorded interview with MUTV, Ferguson confirmed his future plans, which would make him by some distance the Premier League's oldest manager.
The Scot then received a rendition of 'Happy Birthday to you' from the home fans in recognition of his 70th birthday, setting the scene for the slaughter expected to follow.
Except United, injury-hit to the extent of having to name Rafael and Park Ji-sung as central midfield partners, were not up to the task.
The fluency that had carried them to eight wins from nine league games since their only defeat, that 6-1 massacre by Manchester City in October, was so obviously missing.
Nani was an obvious attacking outlet but had one of his frustrating days and neither Berbatov, Hernandez nor Danny Welbeck could plot a path through the massed ranks of Blackburn's defence.
Twenty-four hours earlier, Ferguson had confirmed an intention to hand Berbatov an extra year's contract.
He must have felt like snatching it away, when the Bulgarian followed up his Boxing Day hat-trick against Wigan by hauling Christopher Samba over in the box.
At a club as unstable as Blackburn, one of the few certainties is that from the spot, Yakubu does not miss and the Nigerian promptly sent De Gea the wrong way.
United's response was strange in that, without playing well, they started to create opportunities.
Nani was off target with a couple of efforts then the Portugal winger was denied by Rovers' stand-in keeper Mark Bunn, as was Hernandez.
At one point, Hernandez also got in the way of a goalbound effort from Phil Jones, who had recovered from illness to face his former club for the first time since his £17million move to Old Trafford.
Pilloried by his own fans immediately before Christmas, Rovers boss Kean had now seen his side take first-half leads at Liverpool and United in a staggering six-day spell.
With no Rooney to assist his team, Ferguson turned to Anderson at the break, a move that triggered a number of positional changes, the most significant of which saw Valencia shifted into an advanced position.
But, before United had a chance to see whether the tactical switches would work, they fell further behind.
Yakubu had too much strength for makeshift central defender Michael Carrick as he turned on the edge of the area, then nipped past Jones before drilling his shot through De Gea's legs.
Disastrously for Blackburn, they conceded themselves within 20 seconds of the restart as United advanced, the ball eventually finding its way to Rafael whose bouncing cross-shot was nodded home by Berbatov.
So often have Ferguson's teams recovered from such unpromising situations down the years, Kean must have felt sick.
His side started to get overwhelmed by the tide of red attacks, with Valencia to the fore.
It was the South American's burst of pace and subsequent cut-back that provided Berbatov with the opportunity to side-foot United level.
Minutes later, Bunn almost unbelievably prevented Grant Hanley from turning Berbatov's cross into his own net. He did not know much about it though, as the ball flew over off his outstretched foot.
The weight of attacks on the Blackburn goal was immense. Yet United's commitment was so great they forgot to defend, and once again the questions were raised over De Gea, who came to collect Morten Gamst Pedersen's corner.
Instead, Hanley won the first header, then found the bottom corner with his second.
United resolve was sapped and their hopes of snatching the point that would take them top of the table disappeared when Bunn and home debutant Will Keane got in the way of Jones' injury-time effort.