Also, you mentioned the poop color changing to blackish.
This can be due to the gapeworms passing through the intestine. Gapeworm feed on blood. Black poop is usually indicating the presence of blood.
Other things can cause black colored poop. Chickens that have eaten black berries, blue berries, and especially wood ash or charcoal, will have blackish poop. I just put some wood ash in my chicken's dust bathing areas, now one of my girls is pooping black. She pecks at the charcoal chunks and eats bits of it. Her poop is black off and on, but has good form and texture.
Most likely, the gapeworm are releasing blood as they pass out of the digestive system. Just be vigilant for other symptoms like paleness, lethargy, low appetite, etc.
God Bless