Snapedom

I disagree, I think it is answered in the text. The Unforgivables are obviously Dark Magic, so is 'Sectumsempra' (because its effects are so ghastly, it literally cuts people up).

We also have evidence in the text that more than one definition of Dark Magic exists in Wizarding Britain. Barty Crouch Sr's words in chapter 9 of GOF imply that the Unforgivables are not Dark Arts if performed by Aurors/people who fight Voldemort/people aligned politically with himself/people he likes or something along those lines. And Arthur and Amos stand there listening and do not show any sign of dissent, nor does Arthur say anything critical of Bartemius to the kids later (something along the lines of 'not all those who were against You-Know-Who were also against the Dark Arts'). There is also the curious line by Xeno about the Hallows not being Dark in the crude sense - implying they might be Dark in the non-crude sense. And there even being a non-crude sense of Dark Arts.

Then contrariwise there are the things that are not Dark-Arts that are as bad as Unforgivables but get a pass because they lack the Dark label - love potions, Confundus, Memory Charms. So I join Severus in calling wizards out on their hypocrisy.