Not too hard to figure this out: Your puppy has worms. Call a vet. It can only get worse without intervention.
What Does It Mean When My Puppy Has Soft Black Stool With A Long White Worm?
You need to see your vet to get the appropriate deworming medication for the type of parasite and dosage for your pup. If you can take a fresh, clean stool sample along it can speed things up a bit. Soft black stool may indicate a severe infestation and the parasites are causing damage (bleeding) so I wouldn't put it off. It's not generally expensive unless it is allowed ot become an emergency.