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.
Bastia took the lead in sensational fashion through Julian Palmieri, who beat Nicolas Douchez with a wonderful dipping volley from 25 yards.
Palmieri added a fourth late on to put the seal on a miserable afternoon for PSG, who failed to capitalise on a defeat for leaders Marseille at Montpellier on Friday.
"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."
Blanc insisted PSG could not blame poor fortune for their loss.
"After half-time there was a lot of bad luck on our part, but it is our fault," he added.
