I'm sorry to hear that your girl is having this issue. The dark stool she is having goes hand in hand iwth the abdominal bleed. This is called melena and more can be read here: http://www.petplace.com/dogs/melena-blood-in-stool-in-dogs/page1.aspx As to how long a dog can survive without food, this depends largely on their condition. Fluids are very important, arguably more so than food (as dogs will dehydrate more quickly than one would face starvation). If she will drink, you can try to offer chicken broth without onions as this is a way to get some calories in her. There is also an over the counter supplimnet called Nutri Cal (most pet supply chains carry it) that you can put on her gums. Chicken baby food without onions may also be well tolerated. I hope she's doing better from this soon.
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