The headword is neuter nominative/accusative singular of this adjective, an Homeric epithet. It was generally taken in antiquity, as here, to be a compound from the colour-adjective kelaino/s, 'black, murky' (see kappa 1286, kappa 1285, kappa 1287, kappa 1288) and ne/fos ‘cloud’. See chiefly a scholion to Homer, Iliad 2.412, and cf. (e.g.) scholion D to Iliad 1.397 (cf. ad loc., schol. P.Köln inv.2281); Eustathius On the Odyssey 1.398.14ff.; Hesychius kappa2131 = Anecdota Oxoniensia 1.234.25; Etymologicum Magnum 501.44. Under this theory, a hypothetical *kelainonefh/s has lost an unaccented syllable (Etym.Magn. 501.50ff.) through haplology or syllable-loss through dissimilation (Schwyzer, Gr.Gr. 1. pp.262-633). But Schwyzer himself is unconvinced by this theory and prefers to derive the adjective from an element kelai- (1. pp.447 n.3, 448 Zusatz 2).
It is used in Homer in two senses only (see Führer below), of Zeus, often when invoked in prayer, and of blood (cf. the use of kelaino/n, kappa 1285). It is appropriate to the "cloud-gathering" god of thunder and lightning, and there is no doubt that Homer associates it with storm clouds. But it is open to discussion that the correct etymology has a first element of two short syllables (cf. Dyer below 122-23), giving rise, for example, to *kelenefh/s (‘who commands, moves, speeds the clouds’, first suggested by B. Reiske, Comm. ad Const. Porphyrogen, Bonn 1830, 2.604, on de Cerem. 2.302a), suitable to the lyric metres of prayer but impossible in the hexameter or elegiac metres and requiring lengthening of the second syllable. [Dyer suggests an effect from the laryngeal often posited before ne/fos.]
Of blood (Hesychius kappa2132), the epithet appears little more than a convenient metrical alternative to kelaino/n; cf. Leaf in his edition on Iliad 2.412, "the significance of the second element having been lost." Hainsworth cites it as an example of "modification" (58f.), where "the poet desires to use a certain formulaic word association" (61). The blood that wells from a cut artery may evoke the ominous blackness of storm clouds or clotted blood (cf. schol. Iliad 4.140, "black as a cloud"; cf. 5.798). Neuberger-Donath makes an eloquent case that, in the three instances in the Iliad where cloud-black blood is washed or wiped away (5.798, 14.437, 16.667), the ominous adjective precedes the noun; in the other instance (21.167) the adjective follows the noun and proleptically reminds us of danger in the blood gushing from the wound. In Odyssey 11 it is used twice (36, 153) of the darkening and sinister sheep's blood offered to (and drunk by) the shades. She favours retaining in translation the word association with Zeus of the black clouds. [More prosaically, Leumann and Adrados advanced the theory that Homer had mis-divided a traditional line where the adjective properly belonged to Zeus or Apollo and taken it erroneously with "blood."]
In this way we see something of Homer's poetic use of ambiguous word-associations (see alphaiota 253 and Dyer 126-27). The single word in Homer probably derives from different sources but becomes a useful tool in the poet’s craft.
The word is used after Homer primarily in the Hesiodic Shield and the Homeric Hymns, often in the formula kelainefe/ï *Kroni/wni. Pindar uses it twice of Zeus (Paean fr. 52f = 6.55; 52m.9) and twice innovatively: Pythian 4.52, of the plains of Cyrene, both fertile and loved by Zeus; fr. Hyporch. 108b.3, bizarrely, of darkness.