My hen who is currently molting and looks just awful but is still eating like a horse, has the same type of feces as you just described. very dark and lose and extremely rancid, smells like my cat's. So I would say that is a normal molt symptom. I almost took her to the vet thinking she had a parasite but then discovered upon picking her up, which wasn't easy because she is very grumpy now. But upon picking her up I noticed a huge amount of pin feathers sprouting from her chest and around her neck this morning. So I cancelled the vet, realizing now that she is just molting.