On Dark magic
Dark charms are known as jinxes, hexes and curses. This book does not deal with such spells.
edit: Re my comment to oryx_leucoryx below and whether there may be any difference between "dark" and "Dark": this was at the start of a sentence, so it's unknown whether the capitalization is intentional and meaningful or just conventional.
Well then. It would seem that the official word, consider it actually canon or not as you will (I think Pottermore is in some kind of middle ground, a bit closer to being like the books than like off-the-cuff interviews), is that "Dark" magic is neither necessarily especially "evil" nor only that which is proscribed by society/the Ministry: just things intentionally negative or damaging in some way. (Although looking for some examples I came across Molly remarking in HBP that Ron and Harry looked as if they had had Stretching Jinxes put on them, which doesn't sound quite so bad to me.)
If jinxes, hexes, and curses are "Dark charms", then just about everyone eventually knows at least some Dark magic, it seems to me. There's certainly a number of examples of all the Trio using various jinxes and hexes without this being presented as anything other than normal (and let us not forget Ginny's famous Bat-Bogey Hex).
I this kind of puts an interesting perspective on just what it could really mean to say that Severus is fascinated by or knows a lot of Dark magic.