I'm a fledgeling DM and I used an AL Dark Gifts resurrection on a character in a home game of Curse of Strahd. He randomly got his eyes removed for 60ft blindsight, and at the time I kinda assumed that since he didn't become a bat his blindsight is magical and a 60ft sphere that sees through walls, which I happily announced. Now he spoils me the whole campaign since that kind of sight turned out to be a superpower on par with Spider Sense, even if I say that he doesn't see all of his 60 feet bubble at once. The question is, how do I remove the curse from him logically, beautifully and inoffensively? The guy doesn't abuse it and loves his empty eyesockets, so I don't want to pull a "godly intervention" out of my... Hat.
If you are looking for an "in-game" resolution Ravenloft, and by extension Barovia, has always had a history of twisting things to its liking. The Demiplane of Dread is quasi-aware after all (or at least was in past editions). There is ample room for the environment to adapt to this advantage and thwart it, the walls can become more and more opaque over time to this ability as it adapts.
Another idea is that it could also change to start picking up "false positives". The ability could indicate the presence of things that are not actually there or at least not perceptible to the others but harmless. Like the spirit of a dead girl could set it off as she watches from the sidelines of a battle. He could freak out at "seeing" something more on the battlefield that can cause confusion and mishaps during combat.
I think that Zeiss Ikon has it right that you can just renege on the gift but I agree that an in game solution is usually best for flavor. Then your issue will be when/if they leave Barovia you will have to figure on a "fix" at that time. Or you can just say that the alterations by the Demiplane were fundamental and can't be reversed. Maybe the sensory overload was amplified by the Demiplane so that his brain can no longer process the input through solid objects?
These are not RAW solutions and simply based on flavor and experience over years of dealing with Ravenloft in books and modules. I personally would have limited that to 30ft, but that is my table not yours.
To my eye, the simplest way to deal with this is to admit you made a mistake, and explain to the player that Blindsight doesn't penetrate walls, but rather lets him perceive objects in line of sight -- but independent of lighting or obscuration by magical darkness, smoke, fog, or other non-solid, non-liquid obstacles. You might also explain that it works by subsconscious echolocation, and that magical Silence will obscure his blindsight in the same way Darkness would do for normal vision or infravision. That gives a much more reasonable ability level (overall similar to superior infravision), without requiring undoing the "cool" empty sockets.
To do this without "out of game" activities, you could tell the player that his ability to "see" through walls begins to fade after a (short) time -- seemingly an effect of the magic that is wearing off, leaving him with blindsight as described above after, say, a day, or a week. This could be conveyed in game by describing that what he "sees" through walls is less distinct than it was at first, then later, barely discernible; eventually, he "sees" only the wall, not anything behind it. You could notionally tie this to lunar phases, proximity to the original location where the curse occurred, or whatever else makes sense.
Step 1: Talk to him/her in private, admit that you screwed up and misinterpreted the rules. Put it in the context of "I want to make sure everyone at the table can contribute to problem solving and have fun at the table," and as long as your player is halfway reasonable, he or she should be cool with it.
Step 2: In-game, the character receives a conk on the noggin. After s/he stops seeing stars, s/he realizes that his/her godly sight has faded to mortal levels.
The first thing you have to remember, is that you might be the absolute authority in the world, but that doesn't mean you have to know everything beforehand. It also means the characters only know, what they experience. The character somehow woke up with super-vision instead of normal Blindsight. They might not thought about it, but that is strange. And strange is the trade of adventurers, so make them realize that it is strange. The best way is to frame it like you planned it all along (even if you had no clue or idea about it. It is an ancient DM secret).
Talk to your player about the mechanical effect, but frame it as an opportunity for adventure. ("I feel that your constant 360 wall-hack vision is too powerful, so I will have to change it. Are you up to a cool sotry about it?"). After all a major change in capabilities like this should be a big motivation for the characters, and thus, adventure hook.
DnD is magical place where anything can happen. Use that. Give out ominous clues. Keep them as indistinct as you can (the classics like sour milk, two headed lambs, strange cold winds etc. might work best.) Then, after a night of not so sound sleep, the ability is just gone. No explanation, no saves, no nothing. Let the party try to figure out a cure (go in a dungeon, find a magic item, beat up some wildlife, find a local cure, find the spirit who stole it, etc.) If you feel like somebody playing a blind character for a while would be bad for the game, make a partial cure available.
Maybe the village they are in has a huge catacomb system, where they use ointments of Blindsight. Maybe they find a pendant on an adventurer in a dark tomb, who suspiciously lacks torches. Etc.
So after a session of him not having his sight, after they finish the ritual/find the doodad/kill the boss/free the high priest give him back the normal Blindsight, preferably with the opportunity to use the super-version at some cost (Con damage/Wis damage maybe? I am not the best on DnD rules). That way the he keeps his cool ability, there remains some consistency, but using and abusing it becomes prohibitive because of the cost.
Make it so that he's blind unless he is using an attunement slot to maintain his blindsight.