Sorry Cori's not doing well. When you say "appearing to get sick", are you talking about Cori or his tail? Could he possibly be about to shed? The scales can get darker and then lighten prior to a shed. Has he had an injury to that area, a fall or tail nip?
I'm not sure if this will help or if we are talking about the same thing, but when my beardie was younger, I noticed that his tail was black towards the end. I took pics & talked with some people who said it was normal. He is over a year old & is doing great. lol! I provided a pic just to ease your mind, incase this is simular to what your beardie's tail looks like.
It's stiff? Hmm. Drago's tail never felt stiff. If possible, I would take him to the vet just to have a check up to be on the safe side. I know they can take an ex-ray but that can be kind of pricey & i think they would normally do the it if there were a kink in the tail. I wish I could help more or offer better advice. Please keep us posted.