looks more like from being pecked to me and it's dried blood, I would keep her separated for time being while treated if you can put here in like a dog kennel or partition off a space for here where the others can see her but not get at her, would be best and let her heal up. Then once healed up...
once the bad is out they heal quite quickly, as body isn't fighting the infection so bad to, and she won't go septic on you, Job well done for your hen. Keep us updated
I would try a warm compress held to it to try and help soften it up, it needs to come out asap, I would have some Betadine on hand to clean the area and apply triple antibiotic. I know at the feed store you can get penicillin, not sure on the amoxacillin