Black Hat SEO, as opposed to White Hat SEO, involves the use of techniques that go against the major Search Engines term of service to trick them into giving your website more value than it has, and displaying it higher on the search results for the selected keywords. The use of Black SEO, be it by following bad advice yourself or by hiring the wrong SEO company, will end up getting you penalised on the search engines, or even banned from the listings altogether, and no search engine will accept “I didn't know this was happening” as an excuse. So, how can you detect when your SEO company is using Black SEO, or even if the tutorial you're reading will send you on that path?
The first warning sign is when somebody promises that you'll be first on search results for any keyword, and specially when they claim to have special and secret tactics for that. All the search engines, and specially Google, actively say what they value about a website (though they don't say how much) so there's no special technique there, just good skilled work. Inbound links, good content, good HTML markup, and generally being the best website for your topic or keywords of choice are the only things that work.
Going into more detail, some of the most well known and abused black SEO tactics are:
Your website displays something to search engines (based on IP, user agent or just users that see a text version of the site) and it displays a different website to normal visitors. Usually the website displayed to search engines is stuffed with keywords and links, whereas the one that human visitors see looks normal. This is different from offering a text version and a flash version of a website, which is allowed as long as both have the same content. So creating an accessible version of your pages is good and will increase your SEO, but creating a specially composed page that no human being would like to see since it has no meaning and only keywords is cloaking. And search engines will ban or penalize heavily anybody found using cloaking. They can find out by having your page reported by users, or sending spiders that your website doesn't recognize as a search engine crawler, and eventually they will find out and penalize you. So even if it looks like a way of getting quick benefits and good positioning, keep in mind it will be short lived and crawling back from the banned list will take a long time.
LINK EXCHANGE PROGRAMS OR LINK FARMS
When somebody promises a way of getting a thousand links to your website, check first where those links are going to come from. Link Exchange programs (I link to you, you link to me) will just get a lot of unrelated websites linking to you, and Search Engines check if there's a logical reason why your website is linked to from lots of totally unrelated pages. The effects of this go from just cancelling any benefit coming from those unrelated links to actually dropping your pagerank to 0 since the links to your website aren't reliable. The same happens with Link Farms, just even worse since Google doesn't give any value to them to start with. If you want to invest on a link building campaign, make sure the company you're hiring tells you exactly where the links are going to come from. Might be more expensive, but the results will last longer and be worth it.
HIDDEN LINKS AND TEXT
Using styles to disguise text and links in the same colour as the background (so a human user cannot see them, but search engines can) is also considered a type of cloaking and is definitely Black SEO. Beware of companies adding hidden links to your website, since that will only benefit the receiving website and might make your website an unwilling partner on a Link Exchange or Link Farm program. If you have doubts about your SEO company, check your website pages while disabling the stylesheet.
EXCESSIVE KEYWORD DENSITY
This one falls more on the grey SEO area, and it involves repeating endlessly the targeted keywords so search engines give more value to that page for said keywords. It's not against the terms of service, but search engines consider anything that falls out of normal human language parameters to be bad quality, and so it won't get good results in SEO. Anything that has a keyword density over 12% of the total text is liable to do more harm than good.
If you think your SEO agency is using this kind of tricks to get you quick results, confront them and explain them you don't want your website to be penalized due to using Black Hat SEO. There's many White Hat SEO ways of increasing the traffic to your blog or website without trying to trick search engines and risk being banned from search engine listings.
Using forbidden practices to artificially improve your SERP is known as Black Hat SEO. Those tricks will get you quick results and an even quicker removal from the Search Engine result pages. Learn how to detect Black Hat SEO practices and how to avoid falling for them.
Google, Webmaster guidelines – Webmasters/Site owners Help, Google Webmaster Tools