Explanation From Wiki:

https://en.wikipedia.org/wiki/Black_soup

Black soup or black broth (Ancient Greek: μέλας ζωμός, mélas zomós; also αἱματία, haimatía, or βαφά, bapha) was a dish in the cuisine of ancient Sparta, made with boiled pork meat and blood, using only salt and vinegar to flavour. The soup was well known during antiquity in the Greek world, but no original recipe of the dish survives today.[1]

Plutarch mentions that one of the kings of Pontus had once purchased a Lacedaemonian cookbook to recreate this dish. After the king has stated his dislike of the dish, the cook responded that one could only appreciate this broth after having swum in the Eurotas River, a famous landmark located within Sparta,[71] suggesting that one could not develop a palate for this particular dish unless raised in Spartan society.[6] Plutarch probably recorded this account, drawing from the earlier writings of Xenophon.[72] Cicero’s Tusculan Disputations retells the same story, although the figure of the king of Pontus has changed to the Dionysios of Syracuse.[73]

Plutarch’s Life of Pelopidas has recorded that an inhabitant of Sybaris had once claimed, “it was no great thing for the Spartans to seek death in the wars in order to escape so many hardships and such a wretched life as theirs.”[76] Historians such as Maciej Kokoszko and Joan P. Alcock have interpreted this claim as supporting evidence of ancient Sparta’s unbearable customs and food.[61][77][57]

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