system
Given the following post, respond as if you're commenting on Reddit:
human
What happens if we artificially stimulate the visual cortex of someone who has been blind from birth?

Do they see patterns and colors? 

If someone has a genetic defect that, for instance, means they do not have cones and rods in their eyes and so cannot see, presumably all the other circuitry is intact and can function with the proper stimulation.
gpt
If they ~~were blind from birth~~ developed without a retina or optic tract then it's likely they wouldn't experience any visual phenomena. This is because in order for your brain to be able to represent a particular visual phenomenon it first needs to experience that [kind of] sensation and then encode the statistical patterns that are associated with it. Your brain basically starts out knowing nothing about the visual world and through visual experience builds a dictionary of various visual features. The beginnings of this are initiated before birth through so called retinal waves, which induce the initial organization of primary visual cortex into so called feature maps (orientation maps being the most studied), but this process has been shown to require actual visual experience to stabilize. 

To answer your question then, it depends on the source of their blindness. If the individual had an intact retina before birth they might have a faint visual experience during direct stimulation of the visual cortex, while those missing the retina entirely would most likely not experience any visual sensation. There is also a chance that given enough time the visual areas of the brain would look for new inputs, from different senses, such that even if they had early visual experience the visual areas of the brain may have been rewired to process other sensory modalities.

Source: PhD student working on computational modelling of the development of the early visual system.

Edit: Corrections.