Hello TOpurselover,
Yes, the interior of Saint Laurent dust bags are white. I have also just purchased a Large Envelope bag in Matte Black from Vitkac. My bag appears authentic of my bag, as the bag matches the frommets, interior stamping, etc. As those on my other YSL bags recently bought from other reputable retailers: Saint Laurent, Farfetch, and Selfridges. The grommet lettering on mine appear more straight than yours. But your bag appears to be authentic. But you may be more rest assured if you had it authenticated in the appropriate thread.