If you are concerned you could put the chicken in a cage with paper towels as bedding (food/water) all day to watch if any more stools look like this. Very black tarry stools can be old blood perhaps from coccidiosis or other diseases, but it is hard to diagnose a disease with just poop pictures. Look at how they are eating, drinking, and look for any puffing up or lethargy.
I've posted numerous times in regard to preventative treatment for coccidiosis and internal parasites. I just get tired of repeating myself or have someone interrupt with ridiculous notions that DE or cayenne pepper can prevent or treat those problems, or that by putting dirt from the outside into the brooder will help build immunity. Oocysts can sporulate and become airborne, so tossing a handful of dirt into the brooder does nothing.
From between 2-3 weeks and 7-9 months of age, birds should be given preventative doses of Corid (Amprolium) to build immunity every 3 weeks if medicated feed isn't being used. I use the water method since medicated feeds aren't consistent, they can lose potency sitting on the shelves in feed stores, and people tend to feed their pullets/cockerels other things that make them not consume enough of the medicated feed to build immunity. They then wonder why their chicks get sick.
Birds will get worms. It doesn't matter whether they range or not. If there is moist soil, earthworms, wild birds, etc. there will be worms or worm eggs in the environment that birds ingest. Older birds develop resistance, and younger birds are more prone to them.