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Hello and thank you for using this site to help you. Dark tarry stool means the dog is bleeding inside and you need to waste no time in getting the dog to a vet. Scooting is due to full anal glands which happens when a dog has soft stool.
Here is more on blood in the stool http://www.petplace.com/article/dogs/diseases-conditions-of-dogs/symptoms/melena-blood-in-stool-in-dogs