Every changing stage has its own dress color. This dress symbolise final stage of changing. She is full vampire after being bitten after proposal being accepted (color of her shoes also should be black).
Dress Attributed to House of Worth (French, Date: 1883 Culture: French Medium: silk Dimensions: (a) Length at CB: 15 in. cm) (b) Length at CB: 48 in. cm) Credit Line: Gift of Miss Agnes Miles Carpenter, 1948 Accession Number: b