it is a bad and illegal technique. its better if we all avoid this method. it is against google guideline

In search engine optimization (SEO) terminology, black hat SEO refers to the use of aggressive SEO strategies, techniques and tactics that focus only on search engines and not a human audience, and usually does not obey search engines guidelines.

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Block hat SEO is the does not follow the search engine guidelines it will create spamming for our website and cheat the viewers for posting irreverent content for website traffic.

Black hat SEO is works once upon a time now days went rude with privacy policies in results more of Red hat techniques are making whole your work into spamy world.

Black hat techniques we can simple say that a protocol braking to make things faster and easier without any restrictions.

Black Hat SEO Does Not Follow the Guide Line of search engine.

Keywords Stuffing
Doorway Pages
Hidden content

It means doing work against search engines policies for achieving more organic traffic.

Black hat SEO is an illegal technique. In black hat SEO we violate google algorithm. Black hat technique are making doorway pages, keyword stuffing, page swapping(it means you create a page with different data and when it ranked so you changed the data of content).

As others explained already.

You can use blackhat for your tier 2 or tier 3 websites linking to your money site. This way you will increase the juice flow to your money website.

It is something you should avoid!