It's been my opinion for years that any off-page SEO is blackhat. Search engines don't want people's shitty content at the top of their serps just because they can spam social bookmarking sites and directorys or write articles to get more backlinks... they want valuable content that is going to help their users find what they are looking for.
I'm not an SEO kind of guy, but I have a prediction that the biggest google slap of all is coming gradually over the next few years as their system...