Most of the operations in Blue Pencil include native undo/redo support. This includes strokes created with drawing tools, inserting and removing keys, cut, copy and paste operations, transformations and retiming.
To undo an operation, press 'Ctrl+Z' or select Undo from the main menu (Edit->Undo).
To redo an operation, press 'Ctrl-Y' or select Redo from the main menu (Edit->Redo).
*Hotkeys may vary