Mask forwarded blogger urls to own domain urls

Following Rounin's answer carefully written (thanks a lot) on how to redirect any blogspot urls with any extension to the mydomain.com corresponding URL, now the question is how can I mask the URL? I mean, once the blogspot URL redirects to the mydomain.com, I want to continue to display the original blogspot URL instead of the mydomain.com.

You can use the following JavaScript snippet for that -

<script>
    site = "http://example.com"; // The site which you want to mask, don't add ending slash
    iFrame = document.createElement("iframe"); // Creates a iframe via JavaScript
    iFrame.setAttribute("src", site + location.pathname); // Set the source of iFrame
    iFrame.setAttribute("class", "maskingFrame"); // Add class to iFrame
    document.body.appendChild(iFrame); // Append iframe to body of page
</script> 

And the bare minimal CSS would be -

body {
    overflow:hidden;
}
.maskingFrame, body {
    width:100%;
    height:100%;
    border: none;
}

You can check a demo here (This is the homepage) and here (This is an internal URL from other site which doesn't exist on the original blogspot URL)