Blood that has its source in the stomach is black when vomited up. Your dog may have been poisoned. It's hard to say. Take him to the veterinarian immediately before his condition is irreversible.
You should take your dog to the vet. I have never heard of a dog throwing up anything black before, and I have been around dogs all my life. Hopefully it is not too late.
I would recommend you call a Vet intermediately this could be a reaction to a poison or some type of sickness such as Parvo. If you do not have the money for a vet I am sorry to tell you, you should call the Humaine Society
