#3, "blurring", can be done with a convolution, discussed here:

<http://www.jhlabs.com/ip/blurring.html>

and also available as part of Java Advanced Imaging.

Apple's Mac OS X feature called Exposé darkens background windows when
you press F10 to highlight Application Windows.

<http://www.apple.com/macosx/what-is-macosx/expose.html>

Here's a translucent panel that shows 100% white lettering atop a 50%
transparent black background. You can use this demo to find the value of
the result:

<http://sites.google.com/site/drjohnbmatthews/composite>

import java.awt.AlphaComposite;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.text.SimpleDateFormat;
import java.util.Date;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;

/** @author John B. Matthews */
public class Translucent extends JPanel implements ActionListener {

   private static final Font font =
       new Font("Serif", Font.PLAIN, 36);
   private static final SimpleDateFormat format =
       new SimpleDateFormat("dd-MMM-yyyy HH:mm:ss");
   private final Timer timer = new Timer(1000, this);

   public Translucent() {
       super(true);
       this.setPreferredSize(new Dimension(360, 100));
       timer.start();
   }

   @Override
   protected void paintComponent(Graphics g) {
       Graphics2D g2d = (Graphics2D) g;
       g2d.setRenderingHint(
           RenderingHints.KEY_ANTIALIASING,
           RenderingHints.VALUE_ANTIALIAS_ON);
       g2d.setComposite(AlphaComposite.SrcAtop);
       g2d.fillRect(0, 0, getWidth(), getHeight());
       g2d.setFont(font);
       String s = format.format(new Date());
       int xx = this.getWidth();
       int yy = this.getHeight();
       int w2 = g.getFontMetrics().stringWidth(s) / 2;
       int h2 = g.getFontMetrics().getDescent();
       g2d.setColor(Color.green);
       g2d.drawString(s, xx / 2 - w2, yy / 2 + h2);
   }

   private static void create() {
       JFrame f = new JFrame();
       f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
       f.setBackground(new Color(0.0f, 0.0f, 0.0f, 0.5f));
       f.add(new Translucent());
       f.pack();
       f.setVisible(true);
   }

   public static void main(String[] args) {
       EventQueue.invokeLater(new Runnable() {
           @Override
           public void run() {
               create();
           }
       });
   }

   @Override
   public void actionPerformed(ActionEvent e) {
       this.repaint();
   }
}