United lost 3-2 at Old Trafford.
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."