Chickens have a gross dark colored running poo once a day. It's like a clean out poo. Did you see it once, or multiple times? I highly recommend backyardchickens.com to answer all your chicken questions. If I'm not on FR reading politics, I'm on BYC chatting chickens. I have about 60 birds right now, most in the growing out stages.
Gotta show off my prize Showgirls. The pullet is sitting on eggs right now, due to hatch Monday. Her name is Mirage and her mate is Mandalay, yes after the Vegas hotels.