The champions were 2-0 up inside 20 minutes thanks to goals from Lucas Moura and Adrien Rabiot, only for the home side - who began the day 19th in the table - to battle back brilliantly.
Ryad Boudebouz pulled one back from the penalty spot just after the half-hour mark, before Francois Modesto levelled on the stroke of half-time.
"Having made this start and this dominance with two goals and to completely muffle Bastia, conceding four goals is inexplicable," said Blanc.
"This is a big mistake on our part. We dropped our concentration, our reliability, our rigour.
"Bastia and their fans were woken up by us. At times we have been complacent.
"It's a good life lesson for when we do not do things in the right way. The worst thing in this case is that we are penalising ourselves."
"After half-time there was a lot of bad luck on our part, but it is our fault," he added.
