In old texts pale blue silk was a token of refined taste.
Tips
usage
This literary reading 缥 means a light, pale blue — originally the color of a kind of silk. For the common 'misty/indistinct' sense, use the piāo reading in 缥缈.
Left 纟 is the silk radical 糸 in left-side form. The character originally named a pale silk; in 缥缈 it borrows that wispy, gauzy quality for 'faint and indistinct'.