I am afraid that coughing up blood is not a usual sign for a dog and you may have to take your dog to the vet after all! Was he kicked or had an accident before this happened? Some of the possible reasons for this coming into my mind are heartworms; give your dog heartworm pills for that matter. Since your dog coughed blood only once and he isn’t doing it on regular basis, you can give your dog an over the counter children’s cough syrup. In the end, I would like to reiterate that coughing blood needs a thorough checkup by the vet because there are a variety of reasons for that which can very serious too.