Assuming that the person in question has (physically) perfect sight, but has yet to see even a small amount of light, their eyes would be about as developed as that of a newborn - that is, blurry and confusing. Try crossing your eyes; it suddenly becomes very difficult to focus on anything. Text that was clear moments ago becomes blurry to the point of illegibility. Your eyes didn't get worse; you just shut off part of your eyes' natural, learned ability to focus.
Additionally, even babies in the womb can sense at least a small amount of light. Not much, but some. For someone who has truly seen no light at all, the light of even a dark night may seem painfully bright at first, and the light of day would be crippling, even with closed eyes. The first few hours would be spent simply adjusting to light in general, without even bothering to make sense of it.
Chances are, after some time of adjustment, they would be able to see, but it would take a long time for them to fully match their known information - the shape of an object based on touch - with the information from their eyes. Tiny babies spend hours a day staring at things, training their eyes; for an adult, that time would be difficult, as most adults would like to do something productive, and staring at a wall or their first for 45 minutes just doesn't seem to be anywhere near productive.
This article should be pretty accurate. Quoting from the article:
The absence of these rules can frustrate the newly sighted, whose visual world can be both blurry and two-dimensional—paintings and people are often described as “flat, with dark patches”; a far-away house is “nearby, but requiring the taking of a lot of steps”; streetlights seen through glass are “luminous stains stuck to the window”; sunbeams through tree branches collapse into a single “tree with all the lights in it.” (The writer Jorge Luis Borges, who went blind at age fifty-five, described going blind as a process by which “everything near becomes distant.” In the newly sighted, without depth perception, the opposite seems true: the distant—tiny houses on the horizon, clouds in the impossibly high sky—suddenly looks nearby.)
Without the rules learned from birth, the newly-sighted have to come up with their own rules - starting with the brain figuring out how far apart their eyes actually are. Without that information, it can't combine the left and right images into a single, 3D image. Still, the brain is an amazing thing, and even the elderly could teach their eyes to see.
