Hello, my name is***** and I have over 20 years of experience as a veterinarian.
Of course anyone would be concerned about blood in the stool. The fresher the blood appears the closer the source is to her rectum, so in her case because it was dark it sounds like it was from the proximal colon (closer to the small intestine).
The important factor in all of this is how your girl feels. Since she feels now feels great then that is a good sign.
There is a condition in dogs called hemorrhagic gastroenteritis. These dogs have profuse dark, jelly-clot like, bloody diarrhea. They are very sick dogs and get dehydrated quickly so they need immediate care. Thus if she seems to worsen again, gets very lethargic and/or begins vomiting an emergency veterinary visit is best.
But colitis or inflammation of the large intestine, can be due to stress, a change in diet, eating something she found outdoors, parasites, a viral or bacterial infection, inflammatory bowel disease or even a food allergy. Rarely a mass in the colon can be the cause, or exposure to anticoagulant rodenticides. Another possibility is an anal gland infection. These are scent glands that are located just inside the rectum and an infection can cause a bloody discharge, though should not cause loose stools, and I would expect the blood to still be present.
A raw, irritated colon can take a few days to heal, so the time period this occurred over makes sense. Do you know if she got into anything that may have started all of this?
The fact that she seems to feel much better and her symptoms are lessening makes me think she may be able to work through this on her own. The bleeding should clear completely over 2 to 3 days. If not she should be checked by her regular veterinarian and her anal glands should be checked.
Well worth checking at least a couple fresh stool samples too even if she seems to improve as parasite eggs/cysts are shed intermittently and thus can cause intermittent symptoms and they may be picked up on the second sample or third test if the first is negative.
If she has a tense painful abdomen, continues to have diarrhea, becomes very lethargic, begins vomiting or runs a fever greater than 103.5F then she really must be seen by a veterinarian as soon as possible. Make sure to take a fresh stool sample with you when you go.
But if she seems back to herself I would just have some stool samples checked and take a watch close and see approach.
Please let me know if you have any further questions.