Black combs?

[SuzyB]

Still going through treatment for myco, last girl had third injection yesterday and I thought we were coming to the end of a few bad weeks.

I bring in my pooter chicks each night for a play and Ive noticed that on them, one especially, his/her comb is going black just where it meets the beak. Ive googled and found that black tips on the combs can be a sign of frost bite, but this is at the base of the comb.

Anyone have any ideas what this might be? It not dirt and they seem otherwise in fine fettle, no other outward signs that there is anything wrong.
These are my hatched chicks so Im extra attached to them

[its hilly]

No real ideas. But when they scrap their combs go black where they are pecked, could they be scrapping? Or rubbing their faces?

[RichmondHens]

As Hilly says, could it be blood? Have you tried picking/washing it off to check?