Depends on what the seat is made of.... plastic, mdf, real wood?
Try ruling out any coloured bleaches, toilet-cleansers and/or deodorisers, or other possible spillages from any bathroom substances.... hair dyes, cosmetics, medicines.
Did you recently increase your purchase of blue jeans? I don't wear them much any more, but I used to a lot when I was much younger. The cuffs of my jeans used to turn the top of my white sneakers blue, and after a day of bike riding, the dye would turn my BVD's blue.