Sometimes it helps to see threads of other similar threads. This one is long, but if you scan it, there are a lot of pictures and the wound may be much worse than your hen’s wound:
https://www.backyardchickens.com/threads/serve-open-head-wound-is-black.1291137/
https://www.backyardchickens.com/threads/serve-open-head-wound-is-black.1291137/