If all the cuisine of ancient Greece was characterized by its frugality, derived from the difficult conditions of its agriculture, Spartan gastronomy did not have a reputation for precisely the opposite but for being even stricter and more sober. In fact, some of its dishes were so unpleasant for the rest of the Greeks that they never crossed the borders of Laconia. The Athenian comic poets, for example, used to make fun of them frequently.
The example par excellence is the Caldo Negro, one of the traditional dishes of Sparta and the fundamental basis of its gastronomy and the diet of its citizens. No recipe has been preserved that indicates the ingredients and the amounts used in its preparation. However, from descriptions of ancient sources we know that it was made up of pork meat and blood boiled with salt and vinegar, which resulted in a kind of dark soup with stumbles.
The first mention of Caldo Negro is found in a comedy entitled The miners written by the Athenian poet Pherecrates in the mid-5th century BC, where a woman returning from the underworld claims to have seen black broth flowing through the streets:
All the things of the world there were mixed with wealth and formed with all blessing in every way. Rivers full of porridge and black broth flowed babbling through the streets with spoons and all, and bits of cheese too
Ferecrates, The minersquoted by Ateneo de Náucratis in The Scholars’ Banquet SAW
The plate, which in Greek was called μέλας ζωμός (mélas zōmósthat is, broth or black soup), is also mentioned by Plutarch in his Life of Lycurgusthe mythical lawgiver of Sparta, written in the late 1st and early 2nd centuries AD, in which he states that it was the most famous Spartan dish.
According to Plutarch, the elders limited themselves to eating broth, leaving the meat for the young. Some scholars believe this means that the pieces of meat were to be removed from the soup and served separately to Spartiates who were still of military age.
It also tells the anecdote of a king of Pontus who wanted to try the Black Broth, so he had a Spartan cook prepare it for him. He found it very unpleasant. To which the cook replied that in order to appreciate it, he must first bathe in the Eurotas River, which meant that only those who had grown up in Spartan society were able to taste it (Cicero tells the same story, but about the tyrant Dionysius of Syracuse).
Of all its stews, the most recommended is the black broth, and the elderly do not miss meat, but leave it for the young people, contenting themselves with that broth for all food. It refers to one of the kings of Pontus, who bought a cook from Lacedaemon precisely for this broth; and that having tasted it, he was indignant against him, who told him: “Oh, sir, to taste this broth it is necessary to bathe in the Eurotas!”
Plutarch, Life of Lycurgus 12.4

The comic poet Euphron, who wrote in the third century AD, states when he talks about some famous cooks in his work Brothers that Caldo Negro was invented by a certain Lamprias. Even though it seems unlikely, the truth is that he cites it among others that can be traced in other sources.
Although I have had many students, Lico, you, by your constant good sense and spirit, leave my house a perfect cook, done so in less than ten months, and much younger than all of them. Agis of Rhodes was the only one who could bake a fish to perfection; Nereus of Chios could boil a conger eel to suit the gods; Chariades, who came from Athens, could make a mosaic of eggs with white sauce; black broth came into existence with Lamprias first, Apctonete cooked sausages, Eutino lentil soup, Aristion golden heads
Euphron, Brothers quoted by Ateneo de Náucratis in The Scholars’ Banquet IX
Some researchers believe that the Black Broth was the main dish of the Sisitia, the collective banquet celebrated daily at dusk by the Spartans (the elite of citizens with full rights), in more or less numerous groups. Each of the diners had to pay a monthly fee with which food was bought in the market, and attendance was compulsory, as a means of strengthening and strengthening their bonds of union.
Others believe that it was actually a modest, inexpensive dish that the poorest Spartans brought to the Sysitia. According to Hans van Wees, Black Broth should not be consumed regularly, since it involved the sacrifice of an animal. It would only be consumed when offerings were made, on the occasion of some celebration or designated day.
It is not known if Caldo Negro was also consumed by women and children in the domestic sphere or was something reserved for the banquets of adult men. There is also no evidence to indicate that the Spartans consumed it during their military campaigns.
But it is clear that eating black broth was a sign of integration into Spartan society, as Plutarch expresses when he tells how the Athenian general Alcibiades, when he fled from his family and took refuge in Sparta, tried to integrate by adopting their customs, including eating black broth:
Esteemed, therefore, for his public deeds, and no less admired for his private conduct, he attracted and flattered the crowd by living entirely in the Spartan manner; Well, seeing him with his hair cut to the roots, bathing in cold water, eating puches and tasting black broth, as if they did not believe, and before they strongly doubted that he had ever had a cook, nor had he used ointments, nor had he touched his body the night before. delicate clothing from Miletus. Because among the many abilities he had, it was unique and like an artifice to catch people’s spirits that of resembling and identifying himself in his affections with all kinds of institutions and customs, being quicker to change forms than the chameleon; and with the difference that this, according to what is said, there is a color, which is white, to which it cannot conform, but for Alcibiades neither in good nor in bad there was nothing that he did not equally copy and imitate
Plutarch, Life of Alcibiades 23

The Spartans themselves must have been aware that the Black Broth had a special flavor, not suitable for all palates, and that is why they did not usually offer it to foreign visitors or guests. Once again Plutarch in his Life of Agis and Cleomenes testifies to this fact:
For her daily dinner there were no more than three seats, and she was very spare and very Spartan; but if he invited ambassadors or had guests, then two other seats were put, and the servants used some apparatus for the tables, but not in exquisite stews, nor in pastas, but in taking care that the delicacies were more abundant and the wine was of better quality; so it was that a friend was disfigured by the fact that, having fed some guests, he had put on them the black broth and the cake that they used in their civic banquets: because he said that he had to be careful not to be so rigorous with the guests spartans
Plutarch, Life of Cleomenes 13
Among the modern dishes that can resemble the Spartan Black Caldo is Dinuguan, a Filipino stew made from pork meat and blood, which is simmered and to which garlic, chili and vinegar are added. It is like a kind of blood sausage but in the form of a stew and with the appearance of black soup. And also the Schwarzsauer, a blood soup with black pudding and vinegar typical of northern Germany.
Sources
PlutarchLife of Lycurgus | PlutarchLife of Alcibiades | PlutarchLife of Agis and Cleomenes | Athenaeum of NaucratisThe Banquet of Scholars | Hans van WeesThe Common Messes (in A Companion to Sparta, ed. Anton Powell) | John WilkinsShaun HillFood in the Ancient World | Duncan B CampbellSpartan Warrior 735–331 BC | Eugenia Salza Prina Ricotti, Meals and Recipes from Ancient Greece | Wikipedia