Dark patterns are user interface elements designed to trick people into performing some action they may not have intended.
Google+ offers a single sign-in feature for stuff like blog comments - an example of this can be seen on this blogger post. This is how the comment box is presented to me if I am signed into my Google account:
Now, if you're not paying close attention, you may miss the G+ logo or the 'circles' - it appears to me that this is a text box inviting me to add a comment under the name on my Google account. In actuality, the entire area is a "button" which takes me to a Google+ sign up page. Tricked!
Text only, no HTML, * denotes a required field.