Hart, now widely recognised as England's number one, was involved in a mix-up with Kolo Toure that led to Blackburn taking the lead at Eastlands.
The pair failed to cut out a long ball from Morten Gamst Pedersen and Nikola Kalinic was left with an open goal in the 24th minute.
City responded and equalised in the 55th minute through Patrick Vieira following a neat one-two with Carlos Tevez but could not force a winner.
Reflecting on the opener, Mancini said: "It was a strange goal. It can happen. This is football. Joe Hart is a good goalkeeper."
Skipper Carlos Tevez shrugged off an ankle injury sustained in midweek when he played for Argentina to resume domestic duties, but his touch in front of goal let him down.
Mancini added: "It was very difficult for him as he only arrived the day before the game. But I still thought he had a good match.
"We were very unlucky. We must have had 25 chances to score against Blackburn.
"It is very hard when teams put 10 players behind the ball but we have to try and find a solution."
Source: PA