Before understanding Blackhat SEO Poisoning to the search engines you must understand "What is SEO?" in actual.
SEO or Search Engine Optimization is simply a technique with which ranking of a website can be improved in the Search Engine Result Pages (SERPs) to gain traffic and its visibility.
Most of the search engines such as Google supports Whitehat SEO only and mark the websites as Spam using Blackhat SEO techniques.
Blackhat SEO techniques involves various tricks and techniques with which one can try to jump his website one among the top search results of search engines. But, if this happens i would permanently mark your website as spam so that you can never again get the results back. So, its better try not to fool the search engines. Just be in Queue, and work hard on furnishing your website under rules of the targeting search engine.
What is Blackhat SEO Poisoning?
Its just a cybercrime, as referred by different search engines, done by cybercriminals to fake the results with malicious websites instead of genuine websites.
However, it is increasing to a level somehow but along with this the spiders (crawler, that crawls website and indexes accordingly) of most search engines are getting more smarter that is permanently marking these kind of activities as an illegal offense.
This is referred to as poisoning as it is an illegal act tend to show the users of search engines malicious and unwanted results. Those are even harmful if clicked, so its better to use URL filtering or search engine filtering options.