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Since he arrived at Hogwarts with a knowledge of dark arts, his mother must have taught him, or he learned on his own.
He arrived at Hogwarts with a knowledge of some curses, and this is according to Sirius, so should be taken with grain of salt.
He does say it closely after claiming that Severus was "famous for" being "fascinated by the Dark Arts" (although note that "fascinated by" is not the same as having real knowledge of, nor practicing for their own sake), but it's a separate sentence, and he also takes time to call Severus a "slimy, oily, greasy-haired kid" in between them, so there's some conceptual distance between these utterances as well.
The rest of the sentence in question is saying, essentially, that he was precocious: "more curses ... than half the kids in seventh year", if Sirius is correct and not just exaggerating. A lot of things call this claim into question: How did he know what seventh-years did or did not know? Is he comparing it to what he himself knew at that point? How did he know what Severus did or did not know "when he arrived at school", i.e., when they'd barely met? Unless there's a scene we didn't see where Severus rattled a bunch of stuff off or there was a fight that sounds like it would have been rather one-sided, Sirius has to be assuming based on some later experience (or perhaps just repeating rumour).
Anyway, overall it is not clear that he is claiming the curses were Dark, and even if he is, we don't know that he would be correct to say so. Exactly what constitutes "Dark Arts" is very ill-defined in canon.
Note I'm not saying Severus did not know and practice Dark Arts at any point. He must have, to have been the expert in residence for such situations as the curse on Katie Bell and whatnot. I just think think that he was barely getting started during school, and it was more an intense interest than anything else -- a fan, if you will, rather than a genuinely Dark wizard.
He arrived at Hogwarts with a knowledge of some curses, and this is according to Sirius, so should be taken with grain of salt.
He does say it closely after claiming that Severus was "famous for" being "fascinated by the Dark Arts" (although note that "fascinated by" is not the same as having real knowledge of, nor practicing for their own sake), but it's a separate sentence, and he also takes time to call Severus a "slimy, oily, greasy-haired kid" in between them, so there's some conceptual distance between these utterances as well.
The rest of the sentence in question is saying, essentially, that he was precocious: "more curses ... than half the kids in seventh year", if Sirius is correct and not just exaggerating. A lot of things call this claim into question: How did he know what seventh-years did or did not know? Is he comparing it to what he himself knew at that point? How did he know what Severus did or did not know "when he arrived at school", i.e., when they'd barely met? Unless there's a scene we didn't see where Severus rattled a bunch of stuff off or there was a fight that sounds like it would have been rather one-sided, Sirius has to be assuming based on some later experience (or perhaps just repeating rumour).
Anyway, overall it is not clear that he is claiming the curses were Dark, and even if he is, we don't know that he would be correct to say so. Exactly what constitutes "Dark Arts" is very ill-defined in canon.
Note I'm not saying Severus did not know and practice Dark Arts at any point. He must have, to have been the expert in residence for such situations as the curse on Katie Bell and whatnot. I just think think that he was barely getting started during school, and it was more an intense interest than anything else -- a fan, if you will, rather than a genuinely Dark wizard.