Porch Kitty briefly rallied after her blood transfusion, and my hope was that she was suffering internal trauma. The fact that she'd been losing weight for the past couple of weeks/month didn't really fit, but I chose to ignore that. Unfortunately, her condition began to deteriorate again last night, and it became apparent that she was hemorrhaging into her abdomen still.
My colleague took her to surgery and found what appeared to be diffuse intestinal cancer. The bleeding was from multiple, eroded mesenteric vessels. There was nothing to be done. She was not woken up from surgery.
She will be very, very missed. She was an extremely sweet cat, and she spent her days keeping our neighbor's very ill (terminally) mother company. She was also a favorite of my husband - who worried about her incessantly when she disappeared on her 2 day jaunts.
Rest in peace, sweet kitty.