I don't think this is possible to achieve, or at least not worth $100k, or at least deserving of better than 1:1 payout.
For some precedent:
vsaunce did 3 days in isolation with only light (so the opposite of only darkness but keep in mind it made sleeping difficult) and he nearly quit/had some lasting psychological effect [
https://www.youtube.com/watch?v=iqKdEhx-dD4]
Decades ago an Italian woman set a world record for longest time in isolation at around half a year but she had a custom-built space in a cave with self-adjustable lights, chairs, bed, reading material, and even a computer terminal with only IM to communicate with outside world. She probably could've gone longer and now she only holds the women's record as another guy went a full year since then. [
https://www.youtube.com/watch?v=RYENl-P3Zh8]
I think this challenge could be done very reasonably if the person had reading material and self-controlled light. Even with just self-controlled light so the person can see their food and their toiletries could be a big difference.
One thing that could make it a little easier is without natural light, the body's circadian rhythm greatly expands. The aforementioned 2 people in the caves both thought they were only halfway through their study when they were told to come out after the pre-determined length of time as a result of their day extending to 48 hours (24-36 hours awake, 12-24 hours sleep).