Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you are concerned with the change in color of Carson's stool from brown to black.
The color of a dog's stool is variable but is influenced by what they eat as well as their ability to metabolize the food that they eat, and the metabolic wastes that their body produces. Metabolic wastes can be influenced by organ disease, gastrointestinal parasites, as well as the bacteria population living in their gastrointestinal tract.
In your pup's case with a history of eating and behaving normally, and a new addition of a snack of duck and sweet potato, I am less concerned with his stool color.
Is he having other symptoms such as eating plant material or getting into something he should not have (cat litter box, garbage)?
Any changes in stool consistency, weight loss, increase in water consumption, or lethargy?
His change in diet and subsequent changes in gut bacteria may be entirely to blame change in stool color.
If he seems perfectly normal otherwise then you can try taking a wait and see approach, or adding probiotics like Fortiflora, Proviable, or Benebac to re-establish normal gut bacteria levels and see how he comes along.
The short answer is maybe, a change in the color of his stool could be cause for concern.
But the level of concern will depend upon what other symptoms he is displaying, the consistency of his stools, and the color of his stool (I would be more worried if his stools were black and tarry) as to how concerned we need to be and whether this change persists.
I would mention his change in stool color to his veterinarian, and I would recommend taking a fresh stool sample into his veterinarian parasite check if this persists. I would also recommend an examination if he is developing other symptoms.
But if you stop feeding duck and sweet potato treats and his stool returns to a usual color for him then I would not be overly worried and wouldn't recommend more than submitting a fresh stool sample to his veterinarian for a parasite check.
Please let me know if you have any further questions.