Blood in the vomit does not have to be something serious. For example, if your dog was playing with another dog or chewing on a bone, he could experience a cut in the mouth. This could result in some blood in the vomit. However, if this is a sign of internal bleeding, then that could be a severe problem.
If the source of the problem is located in the esophagus, then you will witness fresh, bright, and red coloration. Other possible sources are the mouth, stomach, and other areas in the upper GI tract. If the blood is darker or appear black, then this means that the blood has already been partially digested and the bleeding has been ongoing for quite a while. Plus, dark blood might have a pungent odor to it. Whatever the diagnosis is, pet owners must bring their dog to the vet for a check-up. It's a good idea to take a picture of the vomit and provide the details to your vet. This can lead to an accurate and speedy diagnosis.