Corrected entry: Just after Draco Malfoy was sorted, Ron mentions to Harry that there wasn't a witch or wizard that went bad that wasn't in Slytherin. However, at this point, Sirius Black was believed to be a dark wizard by the whole wizarding community, and was very well known as well, although he was in Gryffindor when he was at Hogwarts.
Reggmbod110811
20th Oct 2017
Correction: However, Ron is a little kid and heard a rumor that he pronounced as fact. But the adult wizarding community knew better.
lionhead