The Black Hat SEO techniques that are commonly used:
1. Keyword stuffing: how to place a series of keywords on your website
2. Invisible text: How to list a series of keywords to white color font same background of the website
3. Doorway Pages: is the way which people create a page that users can not see, doorway pages created just for the spider can read.
4. Tiny text / Alt text is way which people used to write the text have small size to repeated keywords throughout your website or in the alt tags tag.
5. Duplicate & Mirror sites: often create pages similar in form and content.
6. Submitting repeatedly: regularly submit website to the directories in a short time to be index in the list.