Is it still useful to use black hat and borderline black hat techniques for SEO purposes?

[Mr. Best Answer]

Do search engines block them? Are any of them outright illegal?

A good example is putting a ton of words at the bottom of pages in the same color of the background. So in the case of Yahoo! Answers, it would involve putting white text in the white space. A visitor would only know it existed if they highlighted the entire page; however, there was a time a search engine couldn't tell the difference.
I was more referring to techniques I'm not mentioning. What I used as an example I'd describe as "sneaky".

[Casey Y]

Is that really a black hat technique? I'd say grey hat at the worst.

Regardless, it has been caught by Google and Yahoo at this point, so a waste of time.

[Anita]

Google's anti-spam team are constantly revising the algorithm to filter out web spam... from links created using automated software like blog comments etc, through to hidden text... now days it even goes so far as to "read" your page to check it makes sense and is contextual and so keyword stuffing get detected pretty quick.

What is important to note however is not only will these type of bad links and practices no longer help your rankling, it can actually drop you in the rankings, and in some case get your site sand-boxed (essentially banned).

My advice, steer clear of ALL shady techniques, even if they are not detected now, chances are an update in the future will cost you dearly as recovering from a sanction is a costly process.

Some good tips in the article below, hope this helps..