Thank you so much for the additional information Jim. I really appreciate it.
The dark brown vomit you've described can be a sign of many things, but the big one that we worry about is a bleeding ulcer or a upper gastrointestinal bleed. The fact that she's been vomiting this material for two days is very worrying, so getting her in to see a emergency vet as soon as possible is the key to her making a full recovery.
Since she is a geriatric dog, there's also the possibility she has something like an abdominal mass going on that may have ruptured, causing this dark material to be vomited.
If this had only been going on for a few hours, or if she were a younger dog, I would have recommended fasting her for 24 hours and then feeding a bland die to see if it helps, but since it's been going on for 2 days and she's geriatric, I really feel like the best thing to do is to have her seen by an emergency vet tonight so they can do some x-rays and possible blood work to find out exacly what is going on.
If you're not sure where the emergency vet is in your area, you can go to: XXXXX XXXXX plug in your zip code. They'll tell you where the closest one to you is.
That being said, if you don't have an emergency vet in your area, you can continue to monitor her at home today, but you shoudl absolutely give your vet a call in the morning and have them take a peek at her.
I hope she's feeling beter soon.