Also known as Deceptive Design, Dark Patterns are all about making your user interfaces and user interaction designs as convoluted and bothersome as possible, in an effort to gain something.
Tried to cancel your subscription with a service, but had to hunt all around their website looking for the right page to cancel, only to find that you have to call customer support? That's a Dark Pattern.
Sometimes dark patterns are created by mistake, so it is important to listen to visitor feedback. If you receive word that it was difficult to find or do something on your website, you should try to make it easier or more straight forward. A happy visitor is a converting visitor.
Note: It is unlikely that you will face an algorithm penalty or suffer any negative SEO effects for creating dark patterns, quite the opposite; making it difficult for your visitors to use and navigate your site may just leave to longer session times, technically improving your engagement. Unfortunately, this visitor engagement comes at the cost of frustrating your visitors, which makes it a bad idea.