Simply due to the color of Sundays' beef recipe after it is dried (a dark brown color) some dogs' experience stools that are darker brown than they're used to. A dark brown, even a very dark brown, is completely okay.

If your dog's stool is actually black, especially if its sticky, this is not normal, and may be caused by blood in the stool and you should contact your veterinarian. This is rare and unlikely to be caused by Sundays, but can be caused by severe GI upset, and you should look into it further right away.