Keeping them in the dark is only to make a hen temporarily stop laying eggs, especially when one has suffered a prolapse. They still are only in the dark for 16 out of 24 hours. So, I’m not sure why you were told to keep her in the dark. The calcium or Tums is only to help her try and lay an egg if she has a stuck egg or lash egg. The common signs of avian influenza, which your hen doesn’t have, are face and head swelling, blue combs, gasping, and neurological signs (twisted neck, imbalance, lameness, drooping wings.) You would definitely suspect something is very wrong.