The original poem in Spanish (Galician) reads:
Cando penso que te fuches,
negra sombra que me asombras,
ó pé dos meus cabezales
tornas facéndome mofa.
Cando maxino que es ida,
no mesmo sol te me amostras,
i eres a estrela que brila,
i eres o vento que zoa.
Si cantan, es ti que cantas,
si choran, es ti que choras,
i es o marmurio do río
i es a noite i es a aurora.
En todo estás e ti es todo,
pra min i en min mesma moras,
nin me abandonarás nunca,
sombra que sempre me asombras.
It was written in 1880 (Follas Novas) by Rosalía de Castro
You can read here the English translation that I’ve found in the Internet:
When I think that you have parted,
Black shadow that overshades me,
At the foot of my head pillows
You return making fun of me.
When I fancy that you’ve gone,
From the very sun you taunt me
And you are the star that shines
And you are the wind that moans.
If there’s singing it’s you who sings,
If there’s weeping it’s you who weeps,
And you are the river’s rumour
And the night—and the dawn.
Everywhere you are in every thing,
For and within me you live
Nor will you ever leave me,
Shadow that always shades me.
Theme: The poem reflects Rosalía’s apprehension at the recurrence of sudden misfortune in her life.
Historical background: ‘Negra Sombra’ was probably written shortly after the death of two of her babies, twenty-month-old Adrian who died from a fall in November of 1876 and Valentina who was stillborn three months later (Marina Mayoral: ‘Biografía de Rosalía de Castro.’ Biblioteca Virtual Miguel de Cervantes).
Musical adaptation: The Provincial Museum of Lugo holds the score of the musical adaptation of ‘Negra Sombra’; the sheet of music dates from 1890-1892. Its author was Juan Montes Capón (b. 1840, d. 1899) who composed twenty other pieces, among them another adaptation of a Rosalian poem, Doce Sono (Benigno Lázare: ‘La partitura de Negra Sombra.’ La Voz de Galicia. A Mariña. November 11, 2006).
This poem, ‘Negra Sombra,’ became one of the most emblematic Galician ballads ever when composer Xoán Montés Capón (b. 1840, d. 1899) fused it with an alalá written down in Cruz do Incio (Lugo). The musical arrangement had its debut in Havana’s Grand Theatre in 1892. The ballad is arguably one of the most beautiful and principal in the Galician repertoire; its lyrics so blend with the melody that it is no longer possible to conceive them apart.
Taken from: http://www.rosalia–de–castro.webs.com/Archives/Poems.html#Poem4