I have a pair of sunglasses which have thick brown paper glued to the insides of both lenses. The wearer of the sunglasses is blind when wearing them, but to anyone looking at the wearer it appears they are simply wearing sunglasses. The potential is great! I have Play-toy wear them whenever I am driving her in my car. She gets so anxious when I tell her to lift her dress hem and play with herself. Not knowing who might be watching. Very amusing. And she never knows where I am taking her on any drive. If I pull over for some reason, she immediately gets very scared, even if I have left the engine running.
Well, I have seen these contact lenses on Tumblr. How fantastic would it be to have your slave blinded for as long as you want, in public places, by contact lenses. Much harder to remove than a pair of sunglasses!