could be something she is eating with blue pigment or dye, or something that turns that color when broken down. it would be surprising if it was just one bird though. Blackberries, blueberries, ect? Black poop can be bleeding as well. To find the culprit, you can serially confine them, one by one (or do small groups to narrow down faster)-- this won't work well if it is something she is getting into though- as confined she can't get the item.
Blood was my first thought; if I'd seen this in a human, and knew that they hadn't eaten something with a lot of coloring in it, we'd be on the way to the doctor.
As it is, with just one hen doing it (it appears), I am concerned because they all eat pretty much the same things. I have been feeding them raw peanuts -- 1 c. per 14 hens per day -- to help them through their molting, and they're getting a lot of tomatoes.
We're supposed to have a field trip tomorrow, but I'll take a look at the field & watch the girls to see if I can figure out which one is making the weird poop.