The blind feature will not appear as an option until after you have added other collaborators to the review team.
To activate or deactivate the blind function you have to be the owner of the review, i.e. the person who originally set up the Rayyan review.
Rayyan's voting system can be blinded or not depending on the reviewers preference. For reasons of bias, I would recommend keeping the blind on. This way when each reviewer selects articles for inclusion/exclusion there can be no bias based on what they are seeing other reviewers select.
Simply click on the blind box to activate or deactivate.