Thanks for the solution.I have also tried it on similar lines and it works fine.

There is an client side event called "initializeCombo" in webcombo,  give it any function name say "SetUpEventHandler".

function SetUpEventHandler(webComboId)
{
       var combo = igcmbo_getComboById(webComboId);
 
          if(combo && combo.Editable)
         {
  var box = combo.getInputBox();
  box.attachEvent("onblur()",cboOnBlur);
         }
}

function cboOnBlur(event,controlId)
{
 alert('on blur event');
}