My dogs have been on a raw diet for almost 2 months and are doing very well. My Jack Russell has recently been having stools that are black and a little runny - sort of looks like tar. It's just a litle bit. Most of his stools are hard and whitish in color. My son saw him go to the bathroom the other day and said it looked like he popped a cork out and then squirted out the black stuff. (Sorry to sound disgusting.) What would cause this? In humans, it can mean internal bleeding. What about in dogs?