Basically, The Dark Arts are any science, discipline, magic or martial combat stylenote that is deemed by society in general and the establishment in particular as a Bad Idea.
In other words, if there are Things Man Was Not Meant to Know, you can expect someone to have thought it would be cool to make an easy to read how-to book on it. The "Dark" Art might not necessarily be evil or dangerous, but there's always at least some cause to consider these powers/skills potent and not to be left open for everyone to learn.
So, just who practices the Dark Arts in spite of all these dangers? Lots of people/things. If it is magical, then an evil sorcerer, if it is a science then a mad scientist of course, and if it's a martial art then the Arrogant Kung-Fu Guy is likely to know it. Expect them to learn their craft in the Academy of Evil. If any kind of magic/science is considered to be the Dark Arts, you have Magic Is Evil or Science Is Bad.
Even though the only crime in art is bad taste, the Mad Artist usually knows how to drive viewers mad (in the bad way) with their art. A hero might be able to "redeem" the Dark Arts, or at least use them without becoming damned/addicted when theGodzilla Threshold is crossed.