The following SEO tactics are considered black hat and should not be exercised at all if you want to stay above board with Google and other search engines:
Content Automation
Doorway Pages
Hidden Text or Links
Keyword Stuffing
Reporting a Competitor (or Negative SEO)
Sneaky Redirects
Cloaking
Link Schemes
Guest Posting Networks
Link Manipulation (including buying links)
Article Spinning
Link Farms, Link Wheels or Link Networks
Rich Snippet Markup Spam
Automated Queries to Google
Creating pages, subdomains, or domains with duplicate content
Pages with malicious behavior, such as phishing, viruses, trojans, and other malware
Content Automation
Doorway Pages
Hidden Text or Links
Keyword Stuffing
Reporting a Competitor (or Negative SEO)
Sneaky Redirects
Cloaking
Link Schemes
Guest Posting Networks
Link Manipulation (including buying links)
Article Spinning
Link Farms, Link Wheels or Link Networks
Rich Snippet Markup Spam
Automated Queries to Google
Creating pages, subdomains, or domains with duplicate content
Pages with malicious behavior, such as phishing, viruses, trojans, and other malware