Every blackhat tactic can and will be detected one day. As long as you keep under the radar everything is fine, but you’re bound to get caught at least once in your career. And when you do get caught, you need to make sure only one of your projects will be affected. Search engines like Google are very good at linking different accounts to the same person, so a blackhat SEO needs multiple identities.
How do you hide connections between websites?
So far Google was all about reading what normal users could read. If you made your website accessible to all visitors (including visually impaired), you had a perfect site for Google. Every guideline in the Webmaster guidelines has always been focussed on making a good website for users and forgetting about (spamming) search engines.
But now Google wants us to do the complete opposite. If links are extremely relevant to your users, but not contextual enough to search engines, you need to nofollow them or completely remove them to be able to score. Matt Cutts has written a few blog posts on nofollowing paid links.
What will they think of next? From now on you have to make your website for search engines and hope that it is ok for your visitors?!