[Bo and Spectre]

I have 2 male German Shepherd puppies (6 months old). They are both on a very strict BARF Diet and have been for some time now. They love the food and have thrived on it. My only concern is that they both are pooping what looks like back thick ink. The dogs seem to be ok physically and mentally so I have not gone to my vet yet. Several people have told me that the color is from the dogs ingesting blood from the food, which they do. But shouldn't there be some substance to the stools? Any feedback would be appreciated, thanks!

-Matt in Colorado