blue-black poo

One(?) of my hens is making firm, nicely coiled blue-black poo.  I'm not sure which is/are doing it, but they all eat the same things, so I don't think it's food-color related.

The black color makes me think of bleeding somewhere in the digestive tract.  I've tried to narrow it down to one or two poorly-looking hens, but as all are moulting, they all look poorly!

None of the pics resemble my hen's poo.  I would post a pic if I could, but have never been able to do it.     I shall do my best to describe it to you . . .

The poo looks like a perfect poo specimen, straight from the book.  It's firm, coiled, and has the white urates on the slightly larger end.  The only problem is that the part that is usually sort of grayish-green is blue-black -- imagine a gunstock that's been freshly blued.  And it's really black, not the greenish black your kids' poo turns after they've eaten a lot of black jellybeans!

That's what I was thinking! Blackberries, elder, damsons etc. might cause the colour. You would need to find out which hen/s have a sweet tooth! (I know chooks don't have teeth... )They won't all necessarily fancy fruit.

That's a thought.....

I'm beginning to think that it IS food-related; I'm not seeing any fresh poop, and no one has keeled over yet!    We do have some plants with berries (knotweed and one I can't ident.), so maybe that's it.  I always expect the worst, then I'm pleasantly surprised. 

Thanks, all, for the input.