Check these images against what you are seeing. It’s hard for me to tell. If it is blue mold you can cut a stalk and look for a darkened area of tissue.
Blue Mold | Burley Tobacco Extension
Systemic blue mold can be severely damaging to plants after an outbreak shortly after transplanting. Infected plants may not show symptoms of systemic blue mold until later as plants attempt to grow. Plants will appear stunted and will yellow. These plants will not be productive. A cut through...burleytobaccoextension.ca.uky.edu
It seems to be not exactly the same thing, although there was something brown inside the stem of the only plant I harvested (I will look more cautiously this afternoon). It could be something similar...