Are you in the US? Most state vets can do a necropsy to find a cause of death. Any vet could run a fecal float on some fresh droppings to diagnose coccidiosis or other parasite. Corid or amprollium is available without prescription for coccidiosis, and has no egg withdrawal. Could the blood have been the result of a blood feather or a comb or wattles wound? Many times I have seen a small puddle of blood on the roost or floor from a pecking, and could never find the source. How are your chickens looking now? Are they eating, drinking, and active?