Black hat SEO is both a myth and a reality we have to face sooner or later as SEO practicioners. The sheer existence of black hat SEO techniques must be acknowledged for several reasons.
Hidden text
Create modern CSS based websites with JQuery effects. They often hide large portions of text in layers to display them on click or mouse over for usability reasons.
Cloaking
Hide the heavy Flash animations from Google, show the text-only version optimized for accessibility and findability.
Paid links
Donate for charity, software developers etc. Many of them display links to those who donate.
Keyword stuffing
Tags and folksonomy. Keyword stuff but adding several tags or let your users do the dirty work via UGC tagging every major social site does that.
Mispsellings
Define, correct the misspelled term and/or redirect to the correct version.
Scraping
Create mirrors for popular sites. Offer them to the respective webmasters. Most will be glad to pay less.
Ad only pages
Create all page ads and show them before users see content like many old media do.
Link farm
Create a legit blog network of flagship blogs. A full time pro blogger can manage 3 to 5 high quality blogs by her or himself.
New exploits
Find them and report them, blog about them. You break story and thus you get all the attention and links. Dave Naylor is excellent at it.
you can also hire experts from SEOclerks.