PSG beat Jose Mourinho's team 3-1 on Wednesday evening to give them a commanding lead ahead of the return leg at Stamford Bridge.
However, the Frenchman believes that the tie is far from over.
"It is encouraging," Blanc told reporters. "But qualification is far from being acquired.
"I felt the team was a little dejected [at 1-1 at half time] but I told them that they should return with a fighting spirit. That's what we did.
"The Blues, they did a lot of running [in the first half], so they were struggling. We are really pleased with the second half; that's where we made the difference."