You have some beautiful trim and moldings around those windows. It is both a blessing and a curse since as you say any blind would have to be exterior mounted. I agree with the previous comment that any fabric would be soiled in no time, water splashes far more than we realize. I would have suggested solar blinds if you could have interior mount. If you'd like a pattern you might consider a vinyl version of the roman blind idea. I found an image online which might steer you in this direction. Its a shame to loose your detailed molding, but sun in your eyes in no fun and it might be worth it. Best of luck!
Q