Wisher here is what I could find on blackhead for symptoms:
According to the Michigan Department of Natural Resources and Environment, early signs of blackhead in turkeys include anti-social behavior in the birds, including lethargy. Their heads might draw inward to the body or be tilted downward toward the chest. The turkey's feathers will remain ruffled and the wings will droop. The National Wild Turkey Federation notes that the turkey's eyes might be closed or partly closed and the bird may react if you disturb it, but will quickly become passive again. Blackhead is an old name for the disease, and while the comb of some infected turkeys will turn black, blackening of the comb is not a reliable symptom.
Infected turkeys will stop eating and scratching, becoming emaciated and weak over time. Diarrhea will occur as the disease progresses and the vent feathers will become soiled, according to the Queensland Government Department of Employment, Economic Development and Innovation. As blackhead impacts a turkey's liver, its fecal matter will turn a sulfur yellow color and may appear frothy.
I don't know if you are up to or willing to check this last part but I figured I would include it just in case:
After death, turkeys can be examined internally to verify blackhead infection. Two areas of the bird will be affected, including the ceca, which are two pouches located at the lower end of the small intestine, and the liver. One symptom of blackhead disease includes enlargement of the ceca, which become filled with a creamy cheesy material that ranges in color from whitish green to brown. This material may be streaked with blood and might also have a foul odor. The liver will display areas of dead and dying tissue that look like half-inch yellow-colored rings. Smaller rings will be swollen. As the disease progresses and the rings grow, they may appear sunken and be circled by several rings.
Examination of ceca and liver tissue under a microscope will reveal the presence of the parasite Histomonas meleagridis, which causes blackhead disease.