Mike "The Metal Detector" is an almost completely blind sorcerer.
I'm not applying the blind condition, but I impose disadvantage on anything which requires sight, and auto-fail anything which requires seeing detail.
Early in his training, before he was at a level to control it, he ignored his tutor and tried to cast a high level spell which blew up in his face. His eyes are still there but the heat damaged his retinas and he cannot make out any detail, only discern how bright the room is.
What happens if he casts the Darkvision spell (PHB 230) on himself?
Darkvision is always (to my knowledge) used by creatures who also see normal light. So, what would Mike be able to see, for those 8 hours? I can see some possibilities:
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Mike can see only the additional spectra of light that a darkvision creature can see relative to a non-darkvision creature, although I'm not sure what this would look like.
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It doesn't work at all — darkvision works by upgrading your existing vision functionality and with such badly damaged retinas, the spell fails.
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He gains full vision, including the 400–700nm range, and can see as if he was an orc with darkvision.
What happens?
Best Answer
It'll come down to how you rule your player's near-blindness works.
If you rule that they suffer from the Blinded condition, then Darkvision doesn't negate that. Here's what the Blinded condition states:
Since they can't see it would seem that the vision granted by Darkvision is negated by blindness. The darkvision spell doesn't mention anything about negating the blinded condition - that would seem to be reserved for spells like lesser restoration.
So they'd be under the effects of the darkvision spell, but wouldn't be able to see anyways. The spell could hypothetically be dispelled, not that the subject would notice.
I only bring up the blinded condition because it would make sense that you use that for the mechanical effects of his near-blindness, but if you're ruling the effects of his impairment on a case-by-case basis, you'll have to do the same here. The obvious assumption is that the spell doesn't really help, but since you're firmly in homebrew/dm-ruling territory, the rules aren't going to give you a strict answer.
Part of 5e's philosophy is to empower the DM to make the ruling where there are gaps in the rules. With that in mind, you'll have to decide how close you want to hug the rules for the blinded condition.