jQuery BlockUI Element Blocking showOverlay: false

Question

I am using JQuery BLOCKUI for Element Blocking

but it's showOverlay: false option is not working any idea ??

$("#MessageMOCKUP").block({ css: {
    border: 'none',
    padding: '15px',
    showOverlay: false,
    backgroundColor: '#000',
    '-webkit-border-radius': '10px',
    '-moz-border-radius': '10px',
    opacity: .5,
    color: '#fff'
}
});