The blue star can be found in the safe. To open the safe, you first need to find its combination. You should be able to find the safe combination by inspecting the available items from the screen. If, at any point, you encounter difficulties, press the book icon from the bottom's left corner. By doing this, you will automatically be given a hint for the current puzzle that you are on. If you still can't find a solution for the current puzzle, you can use Google to find a walkthrough or a tutorial for this game.