Black Dusting on some of my hen's combs and waddles (what is it?)

A few of my younger hens have some black discoloration on their combs and waddles. Only a few of them have it and it's only my younger ones that I hatched out at home. They are a mixture of Black Australorps and Golden Comets and Production Reds. Here is a picture. They are all acting fine and eating, they are free range in my backyard and I keep their coop clean. Has anyone seen anything like this before? Should I be worried?