Old thread, but I used toothpaste on mine initially.
It recently went quite blue from ink/denim and I tried a few things including putting it in the dishwasher. I took it out of the dishwasher, still warm, and noticed some had come off. I then rubbed frantically with some quilted kitchen roll (so hard I got a friction burn on my finger!) and all the blue came off. Well pleased. This band is from April 2015.