You could have black wings and a black type dress, with possible sparkles. Black because it was her color and the wings because everyone thought to see her with wings. Sparkles being the stars shining through her.
Thank you for your comment, excellent idea! And maybe some kind of headdress/tiara with stars on it or a necklace made of stars. Ovid described her as having a "poppies garland on her brow", so you could go with that, too. And don't forget a black veil, that's how she was represented in paintings.
In other descriptions, the colours blue and black are mentioned ("a blue-black headband", "The many-eyed embroidery of blue-black Nyx"). She is also described with a "spangled-robed" or as "sable-vestured".
Join in and write your own page! It's easy to do. How? Simply click here to return to Costumes.