Shriveled or darkened combs are either a sign of dehydration or circulatory trouble. Just out of curiosity, I'd open the bird's mouth and look in the back of the throat. Describe what you see in addition to color. If you see yellow plaque looking spots, it is likely Canker. When birds get sick, they also lose weight and it is noticeable on the breast most of all. That's why someone recommended checking the "breast bone". Blackhead is a common disease caused by a protazoa, not a bacteria or virus. Birds get increased thirst, decreased appetite, get weak, and get thin. The same treatment for Blackhead is the same treatment for Canker which is metronidazole 250 mg tab for birds over 5 lbs, and half a tab for birds under 5 lbs. for 5 days. Limit access to earthworms if possible. Do you worm regularly, and if so, what do you use as a wormer?