black ground:

  <div id="mask" style="filter: Alpha(opacity=30); -moz-opacity: 0.3; -khtml-opacity: 0.3;
  opacity: 0.3; background-color: #000; width: 100%; height: 100%; z-index: 5px;
  position: absolute; left: 0; top: 0; display: none; overflow: hidden;">
  </div>

jqurey
 $('#blackmask').show("slow") ;