I don't know, but with what Draco and Sally-Anne said, some people may think it. Though I think it's more likely they'll just think that they mean because the Blacks, Lestranges and Malfoys and all are just maniacs about people being 'worthy' of joining their bloodlines, or being blood traitors in a more general sense.

Do you think Justin will care if strangers suddenly realise he's a muggleborn? I don't think he needs to make a big production of it, either, but he might want to have the option of being the one to say it in public, first.