Ok thank you,
The biggest concern would be an ulcer bleeding in her stomach. Partially digested blood, coming from the stomach, often appears dark or coffee colored.
Check her stool to make sure there are no traces of blood.
Check her gums to make sure they are a nice healthy pink. If they are pale, purple, or an abnormal color she needs to see the vet right away. Abnormal blood color can indicate internal bleeding or shock.
I would continue to monitor her and see if she vomits the dark colored fluid again. If she does, it would be time to see a vet and have blood work done. Kidney
disease is common in older cats. If the kidneys are not working properly toxins build up in the blood and cause nausea and excess stomach acid. Kidney disease can be diagnosed with blood work and a urinalysis.
Pepcid AC is an over the counter human stomach acid reducer. It can be given to cats as well to reducer stomach acid and help settle the stomach. Cats take 1/4 of a 10 mg Pepcid ac tablet twice a day.
Does that help?
Do you have any additional questions?