Not too hard to figure this out: Your puppy has worms. Call a vet. It can only get worse without intervention.
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.