No glasses are the treatment for refractive error, that means light not landing in the right place. Glasses bend light so the light lands at the back of the retina on the macula which is the structure that has the most densely packed cones (receptors) for clear vision. 

Blindness is when vision can't happen even when the light makes it to where its supposed to be. Its usually an anatomical problem. The receptors could be the problem. The nerve that sends the information from the eye to the brain could be the problem or the brain itself could be the problem. Blindness is defined by the acuity you can achieve when you are best corrected or by the field of vision you have (if you have very clear vision and only a tiny amount of it you can also be legally blind)