You cannot blame United for keeping possession. It was up to Blackburn to make them play the ball and they were just so oiutclassed they had no idea what to do
Manchester United do not need the points against Blackpool, but they need to prepare for the Champions League Final. They don't want any injuries or to tire anyone. The team picked for the Blackpool match and the team picked for Barcelona will be very different. This may possibly give Blackpool a chance, but United's second or third team is better than most Premiership club's first team.
It does seem unnanamous though that everyone wants Blackpool to survive.