I wish you luck with your hen.
If you're curious at all, this was my thread with what we think was blackhead. I couldn't confirm diagnosis, so not 100% certain.
I had what I highly suspect was blackhead in one of my really young pullets. No external symptoms other than her sulfer almost neon yellow poop and urate. I humanely culled as she just want not improving at all and was getting worse. Same thing, lethargic, hardly eating or drinking.