Ancient Spartans would only eat flesh, and their black broth was made of animal blood.
Claudius AElianus His Various History
Claudius Aelianus, a Roman author and teacher of rhetoric who lived between c. 175 – c. 235 AD, wrote in his "Various History":
"The cooks at Lacedemon might not dress anything but flesh. He who was skilled in any other kind of cookery was cast out of Sparta. Son of Polybiades, for being grown too fat and heavy through luxury and idleness, they took out of the public Assembly, and threatened to punish him by banishment, unless he altered that blamable and rather Ionic than Laconic course of life: For his shape and habit of body was a shame to Lacedemon and our Laws". - Source
"The dish that was in the highest esteem among the Spartans was called melas zomos, or "black broth" a name which has long excited the curiosity of the learned. What were the precise ingredients of this mess has never been determined with certainty. We remember an old traveler, who, on observing the use of coffee for the first time in the East, conjectured that it was the black broth of the Lacedaemonians! Julius Pollux the preceptor of the Emperor Commodus in his "Onomasticon" says that this famous mess consisted of blood thickened in some particular way. Dr. Lister in his "Notes to Apicius" supposes it was hog's blood; and if so the dish must have had no remote resemblance to the black puddings of our own times. Whatever it was, it could have formed no very alluring dish. We are informed that a citizen of Sybaris having tasted their fare, declared that it was no longer astonishing to him that the Spartans should be so fearless of death in battle, since any one in his senses would much sooner die a thousand deaths than continue to exist on such miserable food. Plutarch relates that a king of Pontus having heard of this celebrated broth purchased a Lacedaemonian cook to make some of it for him. But when he came to taste it he expressed his detestation of the mess in very strong terms on which the cook observed, "Sir, to acquire a relish for this broth it is necessary first to bathe in the Eurotas;" meaning that the hardy habits of the Spartans gave a zest to this fare which it could not otherwise possess. The same writer informs us that the old men were so fond of it that they ranged themselves on one side to eat it leaving the meat to the young people". - Source
Nowadays, it is believed that melas zomos was made of boiled pigs' legs, blood, salt and vinegar. It is thought that the vinegar was used as an emulsifier to keep the blood from clotting during the cooking process.