HI there, this is Dr. Elaine.
How old are these birds?
Layers or broilers?
How many are housed together and in what sized coop?
Do they range free during the day?
What is your worming protocol if any?
The second bird needs to have a post mortem exam. This may be available free to you depending on your location--i.e. the govt (Dept of Ag.) wants to rule out and/or identify any cases of avian flu, West Nile, etc. You must call your local Ag extension office asap. For now, save the body in cool (not frozen) storage.
These girls are a bit old for me to be thinking parasites like Coccidia (which causes bloody--i.e. black stools). However, there are plenty more parasites to be concerned about.
Please take a fresh sample of a "grouping" of fresh droppings from the hens to your local veterinarian. Any parasites can be identified, and although unless you take a hen with you for exam, drugs will not be prescribed, HOWEVER--you will know if you are dealing with an intestinal parasite (possibly on top of something else).
Remove all the pullets from the coop and place them into a clean new containment overnight or tomorrow. Muck out the entire pen--walls, bedding, perches, nest boxes, everything.
Look at NIGHT within the coop to ensure you are not dealing with 'Red Mite'--this is a blood-sucking external parasite that can cause tremendous losses in short order. See the following:
The black coloration you report is often digested blood--i.e. bleeding from the GI tract. Usually this is parasite or infection (like Campylobacter). It is essential you have a stool exam performed.