I agree with Dominique’s definition here – if you use anyone else’s work for whatever reason then it’s black hat.
At the end of the day the white or grey hat definitions is a moot point. Anyone that wants to make money or monetise a website is aware of search engine rankings and what to do to maximise positions for their chosen keywords. So in essence we all try to manipulate the search engines to believe our websites are worthy of a good placement. We might do link exchanges, directory submissions, paid link placements, social bookmarking etc etc. I’ve also seen one particular gambling group that has spawned many “affiliate” sites recently all with thousands of links on dodgy Eastern European sites (low quality and in a short space of time) and low quality content. It’s certainly spammy, but that’s not black hat.
However once you start doing that at the expense of other websites (ie hacking, link dumping, cloaking etc etc) then it’s another ball game altogether.