United fell two behind to a couple of Ayegbeni Yakubu efforts either side of the interval but quickly replied through Dimitar Berbatov, whose brace took his tally to five in two games.
However, as the pressure for a winner intensified, Blackburn broke and when David de Gea failed to connect with a Morten Gamst Pedersen corner 10 minutes from time, Grant Hanley nodded the winner from close range.
"It is a disaster," said Ferguson, whose side failed in their quest to mark his 70th birthday by claiming top spot in the Barclays Premier League.
"I never expected it. We lost two terrible goals and you can't do that."
After successive clean sheets from Anders Lindegaard, questions will now be asked over De Gea, who appeared to have recovered from his early season worries to reaffirm his status as United's number one.
The Spain Under-21 star's culpability for Blackburn's winner seemed obvious, although Ferguson refused to apportion blame.
"We all could have done better," he said, when asked about De Gea. "It was a bad goal to lose."
Ferguson's annoyance was rammed home in even more forceful fashion by former Blackburn defender Phil Jones, who was facing his former club in a fixture he lost 7-1 with Rovers last term.
"I am gutted," he said.
"The manager's birthday as well. We weren't at the races. Too slow, too laboured. Not quick enough. The injuries don't help. But they are no excuse. We were nowhere near good enough."
In fact, Jones felt United had not done themselves any favours by getting back on level terms so quickly.
"That was our downfall, coming back too soon," he said.
"I think we thought we had won it. We got two back, we thought we had won it and we eased off.
"They punished us. They thoroughly deserved their win. We weren't at the races. It will be a bad New Year's Eve.
"I feel worse than when we got smashed 7-1 last season playing for Blackburn. That puts into perspective how disappointed I am."
For Blackburn boss Steve Kean it represented a stunning change in fortunes less than two weeks after it appeared certain he would be sacked after a terrible home defeat to Bolton.
"To come to a place like this and get a result is up there with our best wins," he said.
"As soon as it went to 2-1, we were thinking it was going to be tough. And at 2-2, it would have been a good point."
It flattened the atmosphere around Ferguson's birthday celebrations, which were accompanied by the Scot's confirmation he intends to remain in his job for another three years.
"As long as my health stays up, I would hope to have another three years at the club.
"I am still enjoying it. We have defeats but compared to what we have achieved, they are minuscule."