This reminds me of the tragic story of that 30-some year old man who recently escaped captivity by his mom/step-mom, something like that. Small room, almost no food. He was well under 100 pounds. It took him around 20 years I think, and he had to start a fire to get out. A haunting experience.
Your eyes can adjust to natural darkness, but not to total darkness devoid of all light. Furthermore, her magic is sealed via the collar, and they've likely used magic to seal all exits also. She would have to, without nourishment, remove the collar, gather up enough energy, and then find a way to break through or unlock various magical and physical locks. This is why submission is equivalent to death. It would have been far, far easier to escape before they locked her up and sealed her magic. Don't ever let someone control you like that.