Disqus is a blog commenting system installed on hundreds of thousands of sites worldwide. It allows for blog commenting to be added to any blog or website, however thats not all theyre adding to hundreds thousands of websites worldwide.
WARNING: This article discusses Blackhat Techniques. Blackhat SEO focuses only on search engines and not a human audience, and does not obey search engines guidelines.
Within the embed code for Disqus, a link which is dynamically removed for browsers on load is placed.
This clearly violates the Google Webmaster Guidelines which specifically says to avoid hidden text or links.
This makes Disqus the worlds biggest Blackhat SEO. Disqus have injected their link on over 200,000 sites.
Strictly Speaking: This link alone is not blackhat, many embedded applications link back to their developers websites. However, when a website that utilizes disqus is opened on a browser, the link is dynamically removed from the page for everyone except search engines. This is completed by Disqus Javascript code.
This black hat deception is the sole reason why Disqus.com has a DA (Domain Authority) of 97, which is the same as CNN.
Will Google Punish these actions by Disqus? Only time will tell.