Blackstone and the balloon of death

Though five hundred people witnessed the murder, nobody knows who did it! On the face of it, this is the simplest murder Blackstone has ever had to solve. The entire audience at the George Theatre saw Charlotte Devaraux stab William Kirkpatrick with a fake dagger. They gasped as he fell. But they gasped even more when they realised he was dead. How did the murderer manage to exchange the fake dagger for a real one? What is the source of the exotic poison smeared on the dagger's blade?