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.
Mancini felt it improper to accuse either Hart or Toure for the goal.
"It was a strange goal, " he said. "It can happen. This is football. Joe Hart is a good goalkeeper."