As the name implies, black hat SEO tactics are those that are meant to trick the search engines or site users in an effort to create a shortcut to higher search rankings.
Those tricks can include: using duplicate or low quality content, paid link schemes, cloaking (delivering different content to the search engines and the user), keyword stuffing, and trying to hide keywords on the page (like using text the same colour as the background). All of these tactics violate the guidelines set forth by the major search engines.
They are a bad idea and a waste of time.
The time spent on trying to game the system is better employed in creating a better site (with quality content and a good user experience).
With the release of Google’s Penguin algorithm update in April of 2012, these tactics became even less effective than they had been.
If you are talking with a company about doing SEO and Digital Marketing work for your website, you need to ensure that they will not be using any of these tactics on your behalf.