It is ever-changing, but until recently Google has been cutting out the really obvious, flagrant abuses of the system.
Last year, Google annihilated directories, blog comments and article directories, as well as punishing unnatural-looking link profiles (lots of links from low PR sites, for instance).
They're continuing to refine and expand on this, including certain forms of blog posts in their devaluation net, but more importantly the industry is still catching up.
There are still people out there writing and selling long-form articles, there are still people charging companies to place their links on directories. There is a lag as companies and "SEOs" who are out of the loop fail to realise that their activities are no longer resulting in increased traffic - and then after that lag the SEO's business will collapse.
Many voices in the SEO community are looking at a new paradigm - "no hat" and "true black hat" for want of a better term. The "no hat" spirit is, crudely and generally, "If this wouldn't be useful if SEO wasn't a thing, I shouldn't do it." In other words, good SEO comes as a side effect of good implementation, site design, CRO, social media, content, etc. The "true black hat" meanwhile engages in criminal activities, fraud, hijacks websites and generally takes things on that are inCREDibly risky in an effort to turn a quick buck and disappear.
The old grey and black hats, and to an extent even the white hats, aren't squeaky clean enough for Google any more. At the same time, the loopholes they're trying to exploit are already known and closed (or closing rapidly). So this year, I think this type of SEO will fade away, possibly completely.
I believe that it's possible I'm completely wrong, of course.