Funny you should ask....I just returned from the Vet for the exact same reason,
turns out my own prognosis was correct. After taking X rays we found that there were bone fragments still in the intestines that are a little too big to pass and they have caused some small punctures in the intestinal track.
The black tarry and runny poop which my own dog has been suffering from for the past few days is caused from the blood bleeding into the intestine.
Now he is on an antibiotic and another medication to stop any infection and help break down the bone still stuck in there.
Take your dogs to the Vet, ASAP!
I will feed only softer bones from now on and not until he is fully recovered.