Black Hat SEO is usually classified as unethical - however, i’d think the more appropriate definition would be -
Gaining search engine rankings through the use of methods frowned upon by Search Engines themselves.
To be absolutely candid - Every search engine has a view of what they think users should see when they’re searching for something. This goes hand in hand with the algorithms and methods search engines use to rank websites in search results.
When some website owners and SEO experts use methods that go against what search engines consider ‘ethical’ these people get classified as Black Hat SEOs. And, if and when their methods are detected, their websites fall in danger of being penalized and even banned.
The big danger in using Black Hat methods is that
- your website gets penalized and/or banned.
- you yourself get labeled/flagged as a Black Hat SEO and all of your websites are in danger of being penalized.
Note that the second point is pure conjecture - however, it is based on logic and not whim.