Black hat SEO techniques are unethical strategies used to manipulate search engine algorithms in order to achieve higher rankings in search engine results pages (SERPs).
some common black hat SEO techniques that have been used in the past:
Keyword Stuffing
Hidden Text and Links
Cloaking
Link Farming
Private Blog Networks (PBNs)
Buying Links
Content Automation
Doorway Pages
Negative SEO
Article Spinning