block UI - not working

HTML

<button id="bloc">Block UI</button>

CSS

#blocker
{
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    opacity: .5;
    background-color: #000;
    z-index: 1000;
    overflow: auto;
}
    #blocker div
    {
        position: absolute;
        top: 50%;
        left: 50%;
        width: 5em;
        height: 2em;
        margin: -1em 0 0 -2.5em;
        color: #fff;
        font-weight: bold;
    }
    
    #blocker img
    {
        position: relative;
        top: -55px;
        left: 15%;
    }

JavaScript

//UI block
window.onload = function() {
    
    var blockUI = document.createElement("div");
    blockUI.setAttribute("id", "blocker");
    blockUI.innerHTML = '<div>Loading...<img src="http://www.socialups.com/static/images/fbinventory/ajax_loader.gif"></div>'
    document.body.appendChild(blockUI);
    
    
    var cover = document.getElementById("blocker").style.display = "none";
    
    var btn = document.getElementById("bloc");
    
    btn.onclick = block;
    
    function block()
    {
        document.getElementById("blocker").style.display = "";
        setTimeout(function()
        {
        document.getElementById("blocker").style.display = "none";
        }, 3000);
    }
}