This concoction is included in the menu of the Attic Dinner composed, in a parody of the opening of Homer's Odyssey, by Matron of Pitane (late C4 BC): see Athenaeus, Deipnosophists 4.134D-137C [4.13 Kaibel], at 136E. Elsewhere, though, it appears as a famous -- or infamous -- part of the diet of the classical Spartans. Plutarch retails two variants of an anecdote about it, where either "one of the kings of Pontos" (Life of Lykourgos 12.6-7) or Dionysios (I) the tyrant of Syracuse (Ancient Customs of the Spartans 2) made a point of acquiring a Spartan cook to prepare it for him; the VIP detested the dish but was loftily told that it was appetizing only to those who had bathed in the river Eurotas (epsilon 3710). See also e.g. Plutarch, Life of Alkibiades 23.5: when that renegade Athenian general and hedonist (alpha 1280) fled to Sparta, one of the ways he showed willing in his new, austere environment was to eat the stuff.
Although "black broth" is the standard English rendering of zwmo\s me/las, this may do a poor job of conveying its nature. In consistency it may have been more of a hash or stew, even a sausage; and A.H.M. Jones, Sparta (Oxford 1968) 36, went so far as to call it 'a peculiarly nauseous haggis'. (Compare generally mu 1073.) As to ingredients, those mentioned or implied in various sources include pork meat, salt, and vinegar; but pride of place went to fat and especially, as we see here (in a gloss taken from Pollux 6.57), blood.
For fat and a blood-sausage (xordh\ ai(mati=tis) connected see Sophilos ap. Athenaeus, Deipnosophists 3.125E [3.99 Kaibel]; and cf. generally the Homeric Hymn to Hermes, lines 121-3. (For the possibility that the noun alone, zwmo/s, could imply literal or figurative bloodiness see Theophrastus, Characters 8.7 with R.G. Ussher, The Characters of Theophrastus (London 1960, reprinted Bristol 1993) 93).