Black hat SEO covers any technique that tries to manipulate a search engine's ranking algorithm directly instead of improving a page's actual quality or relevance. Scraping and republishing content from other sites, stuffing pages with repeated keywords, and hiding text by setting its color to match the background so it's invisible to readers but still readable by crawlers are classic examples. Cloaking (showing search engines different content than what real visitors see) and doorway pages built only to funnel traffic elsewhere fall in the same category.
Search engines treat these as guideline violations, not clever workarounds. Google in particular runs both algorithmic detection and manual review, and a site caught using black hat tactics can lose rankings site-wide rather than just on the offending page. The tactics don't even have to be self-inflicted: a competitor can aim spam backlinks or scraped duplicate content at a site specifically to get it penalized, which is why monitoring inbound links matters even for sites that would never touch black hat SEO themselves.
