This is based upon the Latin text and Gregorian melody of one of the five antiphons sung at Vespers for Feasts of the Blessed Virgin Mary. The text, which is from the Latin Vulgate version of the Song of Songs 1:4-5, is as follows:
"I am black and beautiful, O daughters of Jerusalem: therefore the king has loved me and brought me into his bedchamber."
It would surprise me if more than a few of those who sang this back in the good old days of the Latin liturgy ever considered how ironic it was that erotic texts were used to venerate a Virgin (and as a matter of fact, the sexy quotations from the Song of Songs have been replaced with much blander antiphons in the modern vernacular versions of RC Marian vespers/evening prayer). As Dupre's setting amply demonstrates, the actual meaning of the Latin words was not lost on him.