I have the same problem with dry denim on a bianchi celeste saddle. I rode it the other day with thin black shorts that I sweated right through and the sweat and ass friction rubbed the indigo dye right off.
1999 Serotta Csi Custom, ~1984 Pinarello track bike
try softscrub with bleach and use a scrubbing sponge or something with a little bit of abrasive grit. it cleaned my white saddle up a bit, but i still can't get all the discoloration out - it's been too long.
i an old yellow on a bike that's turned green from the blue dye in jeans.