Black hat SEO is easily defined. It’s anything that involves deception in an attempt to rank well in the search engines.
Google has never said not to exchange links. Google has said that excessive link exchanging is bad, many times. If you link to someone else’s site because it’s a good site, and you want to your users to know about it, and that other site also links to you for the same reason, no problem.
Excessive and deceptive link exchanging is the kind of thing you see when people build massive numbers of link pages that aren’t clearly marked or easily found. Keeping those pages more or less hidden from users is a good sign that you’re trying to be deceptive.
The difference is intent, and honestly, it’s pretty easily detected. Good quality sites should never be afraid of exchanging links with other good quality sites, because that’s a natural happening on the web. Good sites link back and forth to each other.
Using someone else’s work isn’t deceptive in every situation either. There is such a thing called “fair use”. I can quote a “reasonable” amount of text from an editorial written on another site if I’m refuting what they’re saying in my own editorial on that subject for example.
Deceptively using someone else’s content would be taking an article from someone else’s site and using it in its entirety, without permission, and the deceptive part is that the scraper is trying to pass himself off as the original author of the piece.
My two cents, anyway.