Most affiliate networks do not approve of cloaking and will disable your account if a merchant complains. Illegal? no. Blackhat is only illegal if there is damage to an online property (from excessive spamming).
Thank you, but how can I know which of them allow cloaking? In their TOS? If I don't read that cloaking is not allowed, does it mean it is allowed? And is there a list of affiliate networks that allow cloaking?
You will never see an affiliate network say they allow cloaking. Chances are the cloaked pages will be deindexed before you get in any trouble with the affiliate networks. Clickbank is the best to use cloaked pages with. I do not recommend using this technique although I have experience with it. For those who do not know what cloaking is: Cloaking is a black hat search engine optimization (SEO) technique in which the content presented to the search engine spider is different to that presented to the users' browser. This is done by delivering content based on the IP addresses or the User-Agent HTTP header of the user requesting the page. When a user is identified as a search engine spider, a server-side script delivers a different version of the web page, one that contains content not present on the visible page. The purpose of cloaking is to deceive search engines so they display the page when it would not otherwise be displayed. As of 2006, better methods of accessibility, including progressive enhancement are available, so cloaking is not considered necessary. Cloaking is often used as a spamdexing technique, to try to trick search engines into giving the relevant site a higher ranking; it can also be used to trick search engine users into visiting a site based on the search engine description which site turns out to have substantially different, or even pornographic content. For this reason, major search engines consider cloaking for deception to be a violation of their guidelines, and therefore, they delist sites when deceptive cloaking is reported. Cloaking is a form of the doorway page technique. A similar technique is also used on the Open Directory Project web directory, for example look at Spreto (The Open Directory Project). It differs in several ways from search engine cloaking: It is intended to fool human editors, rather than computer search engine spiders. The decision to cloak or not is often based upon the HTTP referrer, the user agent or the visitor's IP; but more advanced techniques can be also based upon the client's behaviour analysis after a few page requests: the raw quantity, the sorting of, and latency between subsequent HTTP requests sent to a website's pages, plus the presence of a check for robots.txt file, are some of the parameters in which search engines spiders differ heavily from a natural user behaviour. The referrer tells the URL of the page on which a user clicked a link to get to the page. Some cloakers will give the fake page to anyone who comes from a web directory website, since directory editors will usually examine sites by clicking on links that appear on a directory web page. Other cloakers give the fake page to everyone except those coming from a major search engine; this makes it harder to detect cloaking, while not costing them many visitors, since most people find websites by using a search engine. Source: wiki
Being "blackhat" itself does not make it illegal, just because search engines like google do not like the technique they do not represent the law. Certain blackhat techniques like email spamming and phishing are illegal in certain states and countries like mine Australia.
Blackhat in a nutshell: If it involved tricking the system in any way it is blackhat. Normal SEO does not involve tricking anyone or anything, so it is whitehat. Don't touch blackhat with a ten foot pole--someone is always watching.