Blackie and I went back to the vet today. As I had anticipated, the news is not very encouraging. Naturally they took more blood from him; (he was such a good boy even though it took time to find a good vein). His red blood cell count has not changed - no worse but no improvement. At this point the vet has pretty much ruled out everything that can be diagnosed with blood, poop or urine. Any further testing would involve xrays and that would require anesthesia. Even though I knew the answer, I asked her what could be found with xrays. She said that xrays would show any tumors. If found, then I'd have to decide if I want him to go to a specialist for surgery to remove the tumor, and there is of course no guarantee that removal would help. Before going to see the vet I had decided that I don't want to put him through any trauma just to identify a cause that can't be treated. Blackie hates any disruption in his quiet, routine-filled life. Xrays and surgery would make him miserable, and for what purpose? The vet and I seem to agree on this. She did give us antibiotics - though the tests have been negative for any condition caused by ticks or such, she said it wouldn't hurt to treat for that in case the tests were wrong, or were done too early to test positive. So we'll try the antibiotics and then we'll go back in 2 weeks.
I learned today that bad news is bad news, anticipated or not. But I'm glad he's not in any pain or distress. He's happy to eat, he still enjoys all treats that come his way. And he still wants to walk the trails with me first thing every morning, albeit at a much slower pace.