Below you'll find a list of things that are considered to be black hat.
Hidden Text or Links
Reporting a Competitor (or Negative SEO)
Sneaky Redirects Cloaking
Guest Posting Networks
Link Manipulation (including buying links)
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
Note that all of these are quoted from the above stated source.
However, what made me curious was the mentioning of "Cloaking".
Literally hundreds or perhaps even thousands of internet marketers are cloaking their affiliate links for instance. So what I wonder is, would or could that actually hurt your rankings and is it really Black Hat?