Actually, I have the same thing! I have 2 Buff Orps, a RIR, a Barred Rock, and a Golden Comet, a Golden Laced Wyandotte, and a Silver Leghorn. I have noticed that one egg each day or so is "stained" black or dark green as if ink was on it. Then, I noticed a black, tarry poop on the cement. Voila! Mystery solved, but as a Medical Technologist, I find this very disturbing, is this a lot of blood?
I have it narrowed down to one of the Orps, (they are just now beginning to lay), or the RIR, who has been laying since March. It has been unseasonably hot here in the Seattle area, I go out and they are laying in holes and panting. They have plenty of shade and water, and are eating normally. Everyone is fine in the morning and evening, and they all energetically sqaubble at "snack time".
Thanks for any info!