The blue silk the girl is winding is a symbol of her shallowness and materialism. Silk is a luxury fabric, so its appearance foreshadows the girl's rejection of the rose in favor of more monetarily valuable jewels. The color of the fabric is significant as well, because European artists have traditionally depicted the Virgin Mary draped in blue silk. In this case, however, the use of the color is ironic; Mary's blue robes typically signify her heavenly nature, but the girl in "The Nightingale and the Rose" is entirely worldly.
The The Nightingale and the Rose quotes below all refer to the symbol of Silk. For each quote, you can also see the other characters and themes related to it (each theme is indicated by its own dot and icon, like this one: Note: all page numbers and citation info for the quotes below refer to the Signet edition of The Nightingale and the Rose published in 2008.).
The timeline below shows where the symbol Silk appears in The Nightingale and the Rose. The colored dots and icons indicate which themes are associated with that appearance.
The Nightingale and the Rose
...at her father's (the Professor's) house. When he arrives, the girl is sitting outside spinning silk, and the Student presents her with the flower, saying she will wear it that evening... (full context)