[.net] Semitransparent panel

Hello, I am trying to create semitransparent panel (for example, which is 50% opaque). So I wrote the following code:

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
using System.Drawing; 

namespace Visata
{
   public class TransparentPanel :Panel
   {
      public TransparentPanel()
      { 

      } 

      protected override CreateParams CreateParams
      {
         get
         {
            CreateParams cp = base.CreateParams;
            cp.Exstyle |= 0x20; 
            return cp;
         }
      }

      protected override void OnPaintBackground(PaintEventArgs e)
      {
      } 

   }

}

After this I tried to add TransparentPanel to my form. I noticed that this panel is absolutely invisible. Where is the problem? How to create a panel, which is, for example, blue and semitransparent (opacity is 50%)?. Thanks in advance.