Sooty shrinking comb

[Molnut]

My hens are on pasture all day and locked up at night. They "dust bath" in hole they dig and roll in loose dirt.

I am mentioning this as I noticed that some hens' combs seem soiled with some kind of blackish soot. Their combs seem to be shrinking as well. I tried to rub it off without luck. Maybe I should try to scrub it. Before I stress out everybody (me included), I would be grateful for some feedback.

Any idea what this could be?
Should I give them dust bath areas as well even thought they seem to handle this naturally?

Thanks

 

[Wyorp Rock]

Can you post some photos?
Some people do create a dust bathing area, but that is no guarantee that they will use it especially if they already have a favorite spot.