Sorry - that answer assumed general familiarity with blindsight. Which on reflection may not be common knowledge.
Broadly, the optic nerves feed 2 areas of the brain. One is the visual cortex, which takes up a lot of the brain at the back of the skull and is subdivided into assorted specialist areas that each 'see' 1 aspect of the information from the eye - shape, colour, movement , lines etc. All the info from these visual cortex subsections is then put together into an overall picture and fed to the conscious part of the brain so you know you are seeing a running brown dog, stationary red post box, Mondrian painting or whatever.
But to get to the visual cortex the optic nerves have to go through the middle of the brain, which houses some leftover old bits of brain from earlier stages of evolution. On its way through it drops off some information in the 'obsolete' visual system, but that part of the brain isn't connected to the conscious bit of 'you'. And anyway you have a shiny modern visual cortex to do all the seeing, so that old part doesn't do anything.
Except ....
In some cases people lose vision not because of eye or optic nerve problems, but because of damage to the visual cortex. If the damage Is minor they might still be able to see, but not in colour, or not be able to see where the edges of an object are. If it's severe they might not be able to see at all.
If you ask one of those people with complete sight loss from visual cortex damage to count dots on a screen, or catch a ball you've rolled across the floor they will (understandably) tell you not to be ridiculous, they can't do that because they can't see the dots or ball. If you manage to persuade them to humour you and just guess, they will guess the correct number of dots, and will catch the ball (even if you make sure they cant hear it). But will flat out deny being able to see those things. Which is correct. The 'them' part of the brain can't. The supposedly obsolete bit of the brain still has working 'vision' and feeds the information to the part of the brain that needs to carry out the action, but it doesn't tell the part that knows why or how the body is doing it.