by George Valsamis
Drinking blood means that for a while their body approaches reality, so that they can recognize a living person, they can recognise real life. The blood of the sacrifice, to them is light. Achilles is with the soul of his beloved Patroclus, but he can not be satisfied, since even his friend's soul is confined in and defined by darkness.
I found great help to understand the meaning of this, when I remembered a poem by Takis Papatsonis (20th century). Here is the relevant excerpt:
"… like the eternal night of the wicked death, - fruitless, / without any thing to be feared or expected, without beginning, / without end, unconscious, like the bare (ψιλὴ) concept (ἔννοια) / of death, without trophies of colors, without even the splendor / of a future Judgment (Κρίσεως) with trumpets."
Thinks of this, and remember what Achilles said about the dead, that they are unconscious seblances, think of what Teiresias said to Odysseus, that if they drink blood dead souls will speak the truth, think of Sisyphus and how devoted he was to the vanity of his raising a stone, think of how each dead soul is eternally marked by the last moments of her life above the ground, see how after they drink blood they become aware of their doom, they worry about their relatives, they fear their underground abode…
Homer's Hades is like hell, but not exactly as we know it after Christ, since Homeric Hades is the certain destiny of all mankind without exception.
Reference address : https://www.ellopos.net/elpenor/lessons/lesson3.asp?pg=5