reddit.user.js

// ==UserScript==
// @name           Remove viewed stories & block domains
// @namespace      nothing
// @include        http://reddit.com/*
// @include        http://*.reddit.com/*
// ==/UserScript==

var site_blacklist = /imgur.com|holytaco.com/ig;

function main()
{
    $('head').append('<style type="text/css">a:visited{z-index:666;}</style>');

    //perform action on visited links
    $('.entry > .title a').each(function() {
        if($(this).css('zIndex') == '666')
        {
            $(this).parents('.thing').css({opacity: 0.5, backgroundColor: 'white'});
        }

        if($(this).parent().find('.domain').text().match(site_blacklist))
        {
            $(this).parents('.thing').remove();
        }
    });

    $('.entry > .title a').mousedown(function(){
        $(this).parents('.thing').css({opacity: 0.5, backgroundColor: 'white'});
    });
}


// Check if jQuery's loaded
function GM_wait() {
    if(typeof unsafeWindow.jQuery == 'undefined') { window.setTimeout(GM_wait,100); }
    else { $ = unsafeWindow.jQuery; main(); }
}
GM_wait();