Vets can’t be responsible for recommended herbal remedies like slippery elm. However, it is heartily endorsed by the support groups for people with IBD , Crohns and ulcerative colitis. Bleeding, which is the dark coating, means inflammation, so the mucilage from SE , which is like wallpaper paste, is soothing and healing. No side effects, apart from some slime accompanying the poo. If the antibiotics don’t work, so it’s not an infection, the next step with my dog was a short course of steroids, to reduce inflammation. An endoscopy is a huge step financially, and if your pet isn’t acting in discomfort, is premature. It would only show the location of the ulceration. Then they would give steroids!
I’m not a medical person, but I have family with Crohns, and a previous dog with gut and pancreatic issues. Your pet isn’t losing weight, vomiting or diarrhoea, so don’t despair. You could always try a different vet for a second opinion.