All colored contacts, including those for Halloween, are designed to transform your natural eye color by covering the iris - the colored part of your eye.
Typically, most Halloween contacts focus on altering the iris alone. However, blind contacts like all-white, black, red, or green lenses extend their coverage to both the iris and pupil - the black hole in the middle of the iris to create a truly chilling look.
It's important to note that regular colored contact lenses, which primarily modify the iris, generally do not affect vision. On the other hand, the latter category, with their more extensive coverage, may obscure your vision to some extent.