Just an observation, but may be connected.
I've been keeping my girls in for several days so they don't eat anything other than worming mixture. Last week they had various black splodges on comb tips and sides but all that has gone now. I had, previously, noted that when they have yoghurt and splash it all over then dark spots and marks seem to develop the next day.
I wonder if it's just a temporary reaction to mud, water (yogurt) etc? Sometimes at least.
Just to be clear, I'm describing a black mark like a stain, not a 'spot', leasion or blister etc.