Blind sight, also known as agnosopsia, is a perceptual phenomenon that occurs when people who have blindness due to damage to the primary visual cortex but the actual visual system (the eyes) are undamaged. When this happens in a person they can sometimes respond to visual stimuli that they can 'see' with their eyes but their primary visual cortex cannot translate to the brain. Humans have different areas of the brain that process visual information.
The primary visual cortex is what converts information that we see into the visual mental image that our brain perceives. If the primary visual cortex is damaged but the other areas that perceive visual stimuli are not then blind sight can occur. The sufferer will not be able to consciously 'see' anything because the ability to convert the stimuli into a mental visual image does not exist.
But people with blind sight can still perceive visual stimuli unconsciously because the other areas of the brain are still able to see the visual stimuli. Patients who are fully 'blind' have been able to navigate obstacle courses and show that they can perceive stimuli in lab settings.