Change ALL links, hrefs,urls with vanilla Javascript & regex

After reading your comment, you need something like

function replace_url(elem, attr) {
    var elems = document.getElementsByTagName(elem);
    for (var i = 0; i < elems.length; i++)
        elems[i][attr] = elems[i][attr].replace('blog.domain.info', 'blogtest.domain.info');
}

window.onload = function() {
    replace_url('a', 'href');
    replace_url('img', 'src');
    // etc
}

If you're targeting links, you can do this:

<script type="text/javascript">
    var els = document.getElementsByTagName('a'),
        len = els.length;

    while( len-- ) {
        els[len].hostname = els[len].hostname.replace('blog.domain.info','blogtest.domain.info');
    }
</script>

Place this script just inside the closing </body> tag.

For additional tags, specify the tag name and property you're updating.