It's a virus carried by mosquitos. Usually takes 3 weeks to clear. Harmless enough, usually, and then they are immune for life.
Two things can go wrong, and I see one possibly starting. They can get a secondary infection from the (black) lesions. Neosporin is a good treatment and I'd treat that eye with it. You can ophthalmic ointment from the feed store or the vet -- or you can use plain old Neosporin. Doesn't matter whether it has pain reliever, BTW. The pain reliever you have to watch out for, not use on chickens, is anything ending in "caine" - ex. solarcaine, cetacaine, benzocaine, there are a lot of them. These can actually kill a chicken. But the pain reliever in Neosporin is a completely different drug and I've used it without incident on my chickens.
The other thing is called wet pox -- yours have what is called dry pox. Wet pox occurs inside the mouth and can be serious or even fatal. I'm not even sure whether it is the same virus. If any of your chickens have any problems inside the mouth or start having any respiratory symptoms, research wet pox and treat.
I've only had the dry kind in mine, swabbed most of the black spots with a drop of Betadine, or used Neosporin when it was near the eye to keep the Betadine out of the eye. Only did it once and probably didn't have to do it at all, but felt like I was decreasing the likelihood of a secondary infection. Although I did have one chicken who had a lot of lesions on her head and comb, and got a bit of a secondary infection, and they started pecking her so I had to separate her for a while.
All that was 2009 summer. No fowl pox this summer.
