Black Hat SEO is most commonly defined as a disapproved practice that increases a page's ranking in a search engine result page (SERP). These practices are against the search engine's terms of service and can result in the site being banned from the search engine and affiliate sites. A list of tactics and strategies employed by black hat SEO practitioners have been openly denounced on Google's Webmaster Guidelines and Bing's Webmaster Guidelines.
"Is the work that I'm doing adding value to the user or am I just doing this for search engines to see?" is a litmus test on whether an SEO tactic would go against a search engine's webmaster guideline. If no value is added to the user, but rankings are likely to increase, then your decisions are highly likely to be black hat. The same test can be applied to to paid search practices to determine whether an activity is considered black hat ppc.
Black Hat SEO Tactics
Hidden Text or Links
Reporting a Competitor
Black Hat SEO Techniques
Link Manipulation - Buying links, advertorials, and link schemes
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.