Here's some more clues to the "black swan" and the black tutu.
In a review of Kshesinskaya in Swan Lake in 1901, it states: "The ballerina was very effective in the second act, in her elegant black dress, which went so well on her, and danced the famous pas d’action with aplomb and great artistic finish.”
From Novosti i birzhevaya gazeta (6 Apr. 1901), p. 3. via Wiley, Roland John, The Life and Ballets of Lev Ivanov, Clarendon Press, Oxford, 1997, page 179
Maya Plisetskaya notes this: "The division into the “black” and “white” adagios came to the Bolshoi from the West. The foreign ballet troupes that began visiting at the end of the 1950s reinterpreted Odile, the daughter of the evil genius Rothbart, as the black swan. This division took root."
From I, Maya Plisekskaya by Maya Plisetskaya (translated by Antonina W. Bouis) Yale University Press, New Haven & London, 2000, page 90
Cyril W. Beaumont wrote: "Odile, we are told, is the daughter of Rothbart the magician, but since he makes her assume the likeness of Odette, the expression 'daughter' is more a convenient figure of speech for what is clearly a familiar spirit. That such was the authors' intention is corroborated by the fact that Skalkovsky, describing a performance of Swan Lake at the Maryinsky Theatre in 1899, records that immediately after Siegfried asked Odile - believing her to be Odette - for her hand in marriage, the great hall went dark and Odile changed into an owl."
From The Ballet Called Swan Lake by Cyril W. Beaumont, Dance Horizons, New York, 1982, pages 70 & 71
I also saw a teeny photograph in a French book about Swan Lake which showed Legnani wearing a dark tutu, dark tights, and dark pointe shoes and which identified her as in the role of Odile in Swan Lake. So far I haven't found a larger version of this photograph.