By definition, if the purpose of buying the domain is to manipulate your search rankings then that is black hat. Ryan laid out the only case I can think of that would be an exception to this rule, that is buying a domain that is in your industry or field and redirecting the traffic would be more of a business acquisition then manipulation.
I think there are a lot of people who get tripped up trying to understand the differences between various SEO practices. Why is putting keywords in your title tag, and h1 tags considered white hat when one can argue that you are purposely doing this to manipulate search results, but participating in a link ring considered black hat?
The difference are subtle, the big thing being anything that is done with the sol purpose to trick a search engine into ranking you higher then your site naturally warrants is black hat. Making changes to your site that make it easier for search engines to understand, crawl, or attribute credit to is white hat.
A good analogy would be putting on a nice suit for an interview, this would be considered white hat since you are just trying to present yourself in the best possible manner. (Proper Title Tags, H1 Tags and Page Content, Link Structure, Anchor Text)
However, stealing a new suit, or bribing somebody at the company to say good things about you is considered black hat. (Link Exchange Rings, Buying Links, Buying Social Media, Spamming Forums ect..)
In the end both practices may get you the job so to speak, but more then likely, eventually, the company will find out about it and which practice will you be more proud of? Putting on a nice suit? or paying somebody to say good things about you?