If your vet won't do anything, take it to another vet. This sounds quite serious. Blood in the stool is a sign of internal bleeding, which is never good. Now get that dog to another vet, now!
I agree,,, except I'd go even further and say you need to find the closest emergency vet hospital and take her there.. Your average vet doesn't have the diagnostic equipment that may be needed here... Seems like a life threatening situation to me.