Since when is it common knowledge that Death Eater have a Dark Mark? In GoF even Sirius still wonders what Karkaroff showed Snape on his left underarm. Its quite clear some know it from the moment Snape shows it at the end of GoF. But before?
you know, that’s a good question. it seems impossible to me that members of the order wouldn’t have some idea of the very visible and highly identifiable mark their enemies carried, so one explanation is that sirius wasn’t a full member of the order before he went to azkaban and that’s why he doesn’t know. it’s also possible that the dark marks weren’t common knowledge in the order until the revival of the war during harry’s time, or perhaps they came more into public scrutiny during the death eater trials? (this would be when sirius was in azkaban, so he might have just missed it…?)
it really depends on how much the order (and the wizarding world in general) knew about death eaters, which we don’t really know. i would guess that there wasn’t much common knowledge about them until after voldemort was gone.