As a general rule, blackhat SEO is NOT in and of itself unethical. Sure it tries to manipulate search engine algorithms, but that doesn’t violate any fundamental guidelines of human behaviour. But blackhat practices CAN be unethical.
(Image provided by Stephen T of http://www.locatetv.com/ )
The ethical line is drawn where blackhat SEO significantly and negatively affects humans, and not just some abstract mathematical system.
(I say significantly to avoid people extrapolating that bad user experience is unethical.)
Here are a few instances – hardly comprehensive – where blackhats’ spam is lacking in ethics.
1) Parasite hosting. Pay for your own hosting.
2) Wasting other people’s time because they’re busy moderating your junk comments/submissions etc. This also applies to disrupting the normal functioning of communities by creating massive amounts of spam threads, stories etc.
3) Cracking someone else’s site: you’re using property that doesn’t belong to you.
4) Related to #4, cracking sites in such a way that causes their SERP CTR to drop. E.g. by getting their site labelled as dangerous.
5) Referrer spamming to deliver trojans/viruses/affiliate cookies (e.g. cookie stuffing) to webmasters who you expect will check out their referring sites.
6) Google-bowling a competitor by linkspamming their domain and/or DDOS attacking them and/or cracking their site to have it deliver trojan installers.
7) Stealing other people’s content and republishing it without permission.
At the same time, there are some instances where blackhat SEO is ethical. These typically just take advantage of the limitations of automated ranking systems (search engine algorithms) without troubling humans.
1) Autogenerating content with a Markov Chain or madlib system or other technique.
2) Linkspamming abandoned sites that the owner no longer spends any time on. Of course, this is different than linkspamming someone to the point where they choose to abandon their site…
3) Cloaking your content to rank higher (read:stuffing the cloaked content with keywords; not showing bicycles to SEs and pills to humans, since you’re again stealing humans’ time).
4) Using link farms, doorway pages and/or any other tactics which have no significant negative impact on humans. Doorway pages are kind of borderline since they require an extra click, but I put that more in the category of “crap usability that will kill your conversions” rather than unethical stuff.