I am just getting my flock over fowl pox, it could be that ( no cure ) not fatal unless its wet pox ( inside mouth and down throat ) here is a pic of what my buff orp Bertha looked like.the black spots will fall off in 3-4 weeks then they are immune for life, and yes eggs still eatable.
p.s. iodine will help dry it faster someone told me here and it worked! Not in eyes though. I sure hope your flock feels better.
If you ask me it just looks like caked dirt. If your chickens free range through a garden then don't be too worried. Like the previous post, just try and wash it off. If it is dirt then don't worry. Your chicken won't be bothered by it and you don't have to wash it off.
It does not look like pox. It could be, however doubtful. It looks like a dirty comb, or a comb that is stressed by pecking, biting, lice, etc..wash it with soap and water, apply oil and take another picture. compare the pictures. Hopefully it looks better, cleaner. Spray Neem oil on roosts and that should help.