No blur event fire when leaving editor

I try to use CKEditor with jquery's blur() function. My code look like this :

$(document).ready(function(){
editor = CKEDITOR.replace('content', {
height: ($(document).height() - 150) + 'px',
});


var ckframe = $("#cke_contents_content").contents().get();
$(ckframe).blur(function () {
alert("lost focus");
});
});

But there is no alert message shows when I leave the editor. Did I do anything wrong?