I don't think you understand... The FDA says there are no approved treatments for "food animals." That is true, and it's true because two of the drugs needed to treat it are prohibited for use in poultry.https://www.fda.gov/animal-veterinary/resources-you/blackhead-disease-poultry#:~:text=Birds with blackhead disease are,few days after signs appear.
Unless the treatment you mentioned is recent, I'm not sure one can argue with the FDA...? I mean, i believe you, but just saying. I don't have anything but metronidazole but I will start my hen on that. I already treated her with Safeguard for three days. I would have to order the enrofloxacin, and I'm not sure she will last long enough for me to treat her with that. I learned that Oregano Oil can be very effective as a treatment or preventative. I do have that and will add it to the flock's waterers as hopefully a preventative. I only have one sick hen, but i'm sure it's Blackhead or liver disease.
Metronidazole is an effective treatment for the histomoniasis part of the disease process. The secondary E. coli infections must be treated with a strong antibiotic like enrofloxacin.