This is a very ill defined area, Some people define blackhat differently from others.
Commonly there are 3 hats, White hat, Gray hat, and Blackhat
Googles definition:
White hat: This means that you create a website and you let it sit, and once in a while a visitor will come across your website and think that its interesting. they will then repost the link to your website elsewhere ( Not very likely and very inconsistent if your first starting out) this is why many if not almost all people resort to building links manually.
Google defines any type of artificially link building (You building links yourself or paying other people to build links for you ) as BLACKHAT.
However, it is extremely unavoidable, because the chances that your going to have people reposting your link everywhere on the web naturally is quite slim so that is why most people resort to link building.
The internet marketing blackhat term:
This refers to any type of unethical type of business of work. cloaking, bait n switch, etc.