Yes it is an unethical way of marketing. There's more information at: http://en.wikipedia.org/wiki/Black_hat Best to stick to white hat methods..
you can get get free e-book black hat edition download it here http://cybergold.blogspot.com/2007/05/truth-about-black-hat-marketing.html
Black hat is kind of a waste of time because ultimately all your efforts will be undone once search engines catch on.
thats why black haters are always preparing new new black-hat websites while they wait for the old ones to get banned
Its against legal SEO for example flooding your pages with relevant words so that search engine list your page for that keyword.That keyword is invisible to human eyes only spider can be able to grab.that is one way of blackhat seo.
It can bring u ranked soon and a good traffic can be expected fast.But will get banned when the search engine finds u !
Short term fun...go with black hat Long term success...go with white hat Screw the "gray hat". People never have a clear definition of what exactly you can or cannot do for it to be "gray hat"
What's the point of trying to build a passive, residual income when you're always having to sweat to stay ahead of the search engines?