Cleaning Canvas Window Shades

You are obviously someone who believes in maintaining what you have rather than throwing away and buying new. Good for you. As one who has never seen a canvas window shade (at least I don't think so), only vinyl, I can't really offer any advice on cleaning, but did you ever consider maybe stenciling a design over the dirty area at the bottom or attaching a trim? Canvas would certainly be paintable and you could glue on a trim, which should adhere nicely to the canvas. It would change the look completely. Good luck with your endeavor.