Black hat SEO refers to the use of aggressive SEO strategies, techniques and tactics that focus only on search engines and not a human audience, and usually does not obey search engines guidelines. eg spamming

Black hat link building 

The word ‘spam’ is one that anyone who does any amount of work online dreads. It is, however, generally applied to the masses of incoming e-mail that such people receive every day, whereas there are other forms of ‘spam’ that many such people might not be quite so familiar with. Using some of these is one way that black hat SEO exponents will utilize to generate one-way incoming links. Let is look at some of them now.
Comment spamming Nowadays, no matter what kind of web site you are operating, you need to have a blog attached to it. Indeed, for many online marketing sites, the main ‘home’ page will itself be a blog, because the blog site format allows the owner to add as much fresh, unique new content to that page as they want both quickly and simply. That content is exactly what the search engine spiders love the most, and for that reason, blogs are an essential component of any well regarded, popular web site. Another thing that blogs bring to a site is a degree of interaction between the site owner and their visitors, as all blogs invite comments from visitors that they can add to the blog posts that they read. These have value to the site owner – they indicate that what they are writing merits comment – but they can also have value to the commenter, as they can create a link from the blog to their own site. That is the concept of comment spam, which is where you find as many blogs as you can that carry materials that could be related to whatever type of site you are trying to promote. You then add comments to these blogs as a way of generating links back to your own site. In fact, in what I have described so far, there is nothing at all that is black hat. Where posting blog comments becomes a black hat technique is when you use software to post relatively useless or meaningless comments to blogs using software to do nothing more than create back links. For example, using a program like Blog Comments Poster will let you post lots of comments to blog sites in your niche both quickly and efficiently, while Comment Hut will allow you to find those blogs that you should be posting to. However, while posting masses of blog comments will still generate additional one-way back links for you, it is not as efficient as a link generator as it was a year or two back, for a couple of reasons. First, the majority of blog owners moderate incoming comments before they will publish them on their site. Therefore, unless the comment that you 25 attempt to post is relevant to the subject matter of the post to which it is attached, and unless it adds to the quality of the blog overall, it is unlikely to be approved. Also, the owner of the blog has complete discretion over which comments they approve and those that they do not like, so even if you post high quality comments, they may not always be accepted anyway. The second reason that posting masses of blog comments is not nearly as effective nowadays is because a large percentage of blogs that are added to individual websites use the Wordpress blogging platform, and many of them have the ‘nofollow’ tag enabled. This stops the search engine spiders recognizing or following external links, and therefore the value of any link that is created by posting your comment is vastly reduced. Simply put, posting spammy comments to other people’s blog will get you some incoming one-way links, and they will be valuable, but it will not be so many nowadays.