Black hat is defined as something the search engines don't like, as essentialy they are deceptive. Black hat techniques try to trick the search engines (Google, Bing, and Yahoo) into giving a site an undeserved high ranking. Some techniques are:
Posting junk to forums. You have seen them; the commenter just says something like "Hi" or "good post". They also post links to their web sites. This is why places like Daniweb keep new posters from putting in links.
The specific techniques you asked about, Directory postings, social bookmarking, press releases, classified ads, are not black hat. They can be, if you abuse them. Ask CanadaFred about that.
Really, entire books have been written about black hat SEO. I would go find a book called Search Engine Optimization for Dummies; there is an entire chapter on black hat--so you DON'T do it.
Black Hat search engine optimization is customarily defined as techniques that are used to get higher search rankings in an unethical manner.Some black hat techniques are as follow:-
• breaks search engine rules and regulations
• creates a poor user experience directly because of the black hat SEO techniques utilized on the Web site
• unethically presents content in a different visual or non-visual way to search engine spiders and search engine users.