The Ligue 1 champions beat Chelsea 3-1 in the French capital on Wednesday night to take charge of the tie, but Blanc insists that his side cannot afford to be complacent ahead of the second match.
"We know that whatever the first-leg scoreline, a tough game is ahead of us on Tuesday at Stamford Bridge," the PSG head coach told UEFA.com. "We scored three goals but we saw that we can concede too.
"It's tight with suspense. We'll have to be ready for that test."
Chelsea host PSG in the second leg at Stamford Bridge on Tuesday.