Thats nonsense melanin and vitamin D production is not promoted by exposure to "light". Melanogenesis happens when skin cells are exposed to UV light which the human eye cannot see in the first place. UV light exposure only happens when you are outside as glass blocks UV light. This is like saying blind people can see light because they can feel the warmth of the sun. Even if blind people could sense melanin being generated (which they obviously can't) you would have to expose them to so much UV light that they will feel it by being burnt and not by sensing it on some mystical cellular level. And Vitamin D? that's even more far fetched than the melanin theory...

Some clinically blind people with no eye damage do maintain the ability to distinguish between light and dark, that's due to certain cells in the cornea which are light sensitive and still maintain a neural connection to the brain.

People who are legally blind say a really bad case of cataract sill have light vision and usually can identify shapes and see a very very blurry and narrow image of their environment but those are not clinically blind.