"Hopefully it was just in that bud because it was so damn thick and dense."

This is almost exactly the problem. Despite the external humidity, the humidity relative to the inside of that bud was much higher, pus the darkness made for a perfect mold environment.

When I grow big stuff like that, I always make sure to get a couple of bamboo skewers and try to separate the buds out a bit to allow more airflow and less chance of bud mold before harvest.