Derek left a comment regarding surreptitious BDSM equipment relating to contact lenses which are made as blindfolds. A fantastic idea but I have heard stories of fancy dress contact lenses causing damage so could never myself take the risk of using these BDSM lenses on a precious sub of mine. And there is the problem that only an experienced contact lense wearer is likely to be able to wear contact lenses free of streaming eyes. The notion did however get my creativity going for using medical grade contact lenses to ‘blindfold’ a sub who is a regular contact lense wearer, based on my own experience of wearing contact lenses. Simply use a prescription specification that is at an extreme end of what is available from proper optical suppliers.
Base curve and diameter can match the sub’s existing prescription but the power can be the opposite of what they need. Given the power range available is from -12 to +6, I imagine it would be possible to use lenses which would leave the wearer seeing no more than a blur of light and very vague shapes. The fun that could be had! If the sub has a prescription of +3.5 but was made to wear lenses of -12, I am sure it would work and I think be safe for shortish periods?