An extensions that interacts with javafx-scenebuilder
In your BlueJ Project directory place bluej-scenebuilder.jar (found in /library/ folder) into a folder 'extensions2'. BlueJ needs to be restarted for the extension to load.
In the BlueJ settings dialog you will find a tab called 'Extensions'. There you can set the path to your JavaFX-Scenebuilder executable. On Linux this could be "/opt/scenebuilder/bin/SceneBuilder".
Any .fxml file in your project folder will displayed next to your class diagram. With "double click" or "right click -> open" the file will be opened to be edited in Scenebuilder.
You can also create a new .fxml file or delete an existing file from the context menu.