Hi, it sounds like it could be blood because you mentioned the colour was mostly black and reddish brown. I would wait until his next poop, and place the sample in a baggy and take it to your vet to be tested. Your vet will be able to test it to see if there is any blood in the stool. I wouldn't wait for this because if he is bleeding internally this could potentially be a medical emergency. I hope it's nothing serious, and just that he has a irritation in the bowel, but I would have it checked out to make sure. Please keep us posted.