How to use Java AWT setBackground

You have added the JPanel over the JFrame which completely blocks out the underlying container on which you have set the color.

You could do this instead:

public Gui(String title) {
   super(title);
   JPanel pane = new JPanel();
   setBounds(100, 100, 500, 500);
   setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   Container con = this.getContentPane();
   pane.setBackground(new Color(0, 0, 0));
   con.add(pane);
   setVisible(true);
}