O que a “mancha na lua” na Canção de Durin se refere?

Esta é provavelmente uma licença poética da parte do criador da música; É difícil reconciliar os rascunhos de Tolkien com uma situação em que a Lua está no céu, mas não manchada, antes do despertar dos Anões, e é então manchada mais tarde. Mas podemos assumir uma interpretação literal e, na verdade, faz mais sentido do que você esperaria, embora não seja perfeito.

Que "mancha"?

Se se refere a alguma coisa, provavelmente se refere a este evento (ênfase minha):

Now Varda purposed that the two vessels should journey in Ilmen and ever be aloft, but not together; each should pass from Valinor into the east and return, the one issuing from the west as the other turned from the east. Thus the first of the new days were reckoned after the manner of the Trees, from the mingling of the lights when Arien and Tilion passed in then- courses, above the middle of the Earth. But Tilion was wayward and uncertain in speed, and held not to his appointed path; and he sought to come near to Arien, being drawn by her splendour, though the flame of Anar scorched him, and the island of the Moon was darkened.

^{The Silmarillion III Quenta Silmarillion Chapter 11: "Of the Sun and Moon and the Hiding of Valinor"}

Esta história não entraria na narrativa até depois que O Retorno do Rei fosse escrito, por isso é difícil conciliar.

Mas ... que lua?

Interessante notar é que existem versões do Legendarium em que a lua antecede o despertar dos anões; Em um desses rascunhos, que Christopher Tolkien data por volta de 1946-1948, a lua é, na verdade, um produto da petulância de Melkor:

[T]he Earth may not be wholly destroyed against its fate; nevertheless Melkor took a portion of it, and seized it for his own, and reft it away; and he made it a little earth of his own, and it wheeled round about in the sky, following the greater earth wheresoever it went, so that Melkor could observe thence all that happened below, and could send forth his malice and trouble the seas and shake the lands. And still there is rumour among the Eldar of the war in which the Valar assaulted the stronghold of Melkor, and cast him out, and removed it further from the Earth, and it remains in the sky, Ithil whom Men call the Moon.

^{History of Middle-earth X Morgoth's Ring Chapter 1: "Ainulindalë"}

Além disso, versões anteriores do despertar dos Anões colocam sua primeira aparição na Primeira Era, após a criação do Sol e da Lua:

104. About this time the Gnomes climbed Eredlindon and gazed eastward, but they did not pass into the lands beyond. In those mountains the folk of Cranthir came first upon the Dwarves, and there was yet no enmity between them, and nonetheless little love. It was not known in those days whence the Dwarves had origin, save that they were not of Elf-kin or of mortal kind, nor yet of Morgoth's breeding.

^{History of Middle-earth IV The Lost Road and Other Writings Part Two: "Valinor and Middle-earth before The Lord of the Rings" Chapter III: "The Later Annals of Beleriand"}

Não há nada dito sobre quando os Anões acordaram pela primeira vez, mas pelo menos admite a possibilidade de que foi depois do surgimento da Lua.

Há uma breve referência no Silmarillion à lua tendo sido "enegrecida" pela chama de Anar (literalmente, a luz do sol)

Thus the first of the new days were reckoned after the manner of the Trees, from the mingling of the lights when Arien and Tilion passed in then- courses, above the middle of the Earth. But Tilion was wayward and uncertain in speed, and held not to his appointed path; and he sought to come near to Arien, being drawn by her splendour, though the flame of Anar scorched him, and the island of the Moon was darkened.

No entanto, é razoavelmente claro que o significado de Tolkien na Sociedade do Anel é simplesmente que a lua era muito jovem e, portanto, imaculada por crateras e impactos.