Good evening, thanks for your question. I'm sorry about the delay, there are a limited number of experts on this evening so it's taken us longer than normal to get to all of the questions.
Anytime there is blood in the stool of an animal, we become concerned. In a 10 year old dog, we would want to rule out a parasitic infection with a fecal exam (hookworms and whipworms commonly cause both diarrhea and diarrhea with blood in it), pancreatitis (commonly from eating human or fat-laden foods), HGE (hemorrhagic gastroenteritis), etc. It would be advisable to present him to a veterinary office soon and have a fecal exam, CBC, Chemistry, Electrolyte and Pancreatic Lipase tests run to see what may be causing the stool problems. Your vet will likely place Spock on a regimen of antibiotics, a GI protectant or antacid and possibly an antiemetic if you see any vomiting before presentation. If dehydration shows up on the lab work, your vet is likely to recommend either SQ fluids (fluids under the skin) or IV fluids--all depending on what level of dehydration is present.
In the meanwhile, feed Spock a gentle diet of white or brown rice boiled with just a plain, skinless chicken breast. Do NOT feed any more milk. Milk can not only cause diarrhea but can worsen the symptoms Spock is already having. If you haven't already, cut out ALL human foods (this is the leading cause of bloody diarrhea in adult dogs). Your vet may wish to do a few days worth of a gentle GI diet such as Hill's I/D or W/D after you've presented him to the clinic. They may also instruct you to keep up with the rice and chicken instead.
If my answer has satisfied your question, please take the time to leave positive feedback. This is how experts are compensated for their time with each customer. If questions remain, reply and I will assist you further. Best of luck!