Black Hat I believe is a general term applied to general nefarious and dubious methods to obtain some kind of digital access or advantage. Whether it's black hat hacking and breaking through computer security systems without permission, to black hat SEO, which generally includes doing things to game search results in a way that is against Google's terms and conditions. Black hats techniques in SEO can also be very profitable. Think about how often you click on the second or third organic search result on Google, that's provided you know enough to skip the ads (most people don't). I'm guessing it's quite rarely in comparison to the first spot. If you're a company that sells a product or service, the top spot can be the jackpot for your business (provided your website is good enough to convert visitors once they land on it). As a company you have some input on the ad section, you can simply buy ads to ensure you're taking up one of those spots. If you're a local business, you then have the map pack to contend with, which can be manipulated to a certain extent using Google My Business and ensuring you're getting a constant stream of well written good reviews, only then do you get the normal organic listings.Therefore some companies are willing to pay top dollar to SEO companies to get them to the top spot in the organic listings. Black Hat SEOs often go for short term, high risk high reward tactics that could end up hurting the client in the long term if google finds out and penalises their site. One technique is using something like a PBN (private blog network) that they've built to give the clients site fake backlinks. This is risky though, as if the PBN is detected by google then all sites linked to it will loose the PBN link juice and risk getting heavy penalties that the site may never be able to recover from.