Darkling I stood

I'm going to look into this: I've a hunch Keats had read the Odes of Pindar, in their translation by Gilbert West (1749), possibly in their augmented version by Francis Lee, chaplain to the Prince of Wales [Francis Lee, The Odes of Pindar, in Celebration of the Olympic, Pythian, Nemean and Isthmian Games, Translated from the Greek; not one fourth of which have ever before appeared in English. Including those of Mr. West (London 1810)], prior to writing his 'Nightingale' Ode in 1819.  I'm thinking particularly of West's version of the first Olympic Ode, lines 141f. (this is the fifth Antistrophe):

Anxious then th’Elean bride
From her royal sire to gain,
Near the billow-beaten side
Of the foam-besilver’d main
Darkling and alone he stood,
Invocating oft the name
Of the trident-bearing god:
Straight the trident-bearer came.

Lee glosses: the bride is 'Hippodamia, the daughter of OEnomasnus, king of Pisa, who being extremely fond of his daughter (the most beautiful woman of her time), and therefore unwilling to part with her, obliged every one who sought her hand in marriage to contend with him in the chariot race.' It's not just the 'darkling he stood' as a precursor to Keats's 'darkling I listened' -- although it seems to me likely that's where junkets got the phrase from -- but the perilous foam.