You could give Corid and Ivermectin...but why?
Ivermectin is not an antibiotic. It won't touch cecal worms (the vector of blackhead). Keeping cecal worms down helps avoid the transmission of blackhead form the cecal worms, but worming now won't cure the protozoa in the bird's system...if that is what is is.
There is nothing on the market now for poultry with histomoniasis (blackhead). Some antibiotics help, but it is thought for the secondary infections.
Anything to support the liver would be good...I've never heard of nor used Phyllanthus niri, so I can't comment on its effectiveness nor safety for chickens. Note, not all herbals are safe for hens. Notably, thyme can be toxic.
As to Corid...that could be helpful. I have seen some commentary that has seen yellowish poop with coccidioisis. Coccidiosis does not have to produce bloody diarrhea. It depends on the type of coccida that is causing the infection. Some nestle higher in the digestive tract, causing stress on the liver.
Corid would be an option, as well as some other general support.
It is good that she is eating and drinking and acting normally. It may be a low level infection or possibly some internal issues. That is a problem with the aging commercial breeds.
Good luck. Hopefully she will rally.
Lofmc