Black tar like stool is very concerning and really warrants a vet visit especially in a senior dog. This indicates bleeding in the digestive tract most often the upper portion of the intestinal tract.
This can be due to many things which is why a hands on exam is needed. The most common causes would be
* Eating something toxic such as rat poison
* Blockage in the digestive tract due to something she has eaten or a tumor
* Problems with the clotting factors of the blood
* Ulcers in the stomach.
* Complications from heat stroke
If he gums are pale in color then I would get her in to the ER tonight.You mention her stomach is red if this looks like bruising then this too requires immediate care.If they are still pink you can monitor her for the evening and have her seen tomorrow but she does need to be seen.
For tonight you can try Pepcid AC in case this is due to ulcers in the stomach. She can have 1/4 of a 10mg tablet every 12 hours.
You can try offering her a small amount of boiled chicken and see if she will eat that but I would not force it.
The following link goes over this in more detail LINK
I am sure you were hoping for an at home treatment and I always try to do that when possible. However with these symptoms and her age at home treatment and waiting would not be in her best interest. If at all possible I would have her seen tonight. I am concerned that if the cause is not found and treatment started with in the next 48 hours her chance of full recovery will be really diminished.
Please keep me posted on how she is doing. Please reply back if you have additional questions and I will be happy to continue.
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