Mrs. Stewart's Liquid Bluing. It's in a small blue bottle with a white label of an old lady on the front. It's available in the laundry supplies aisle of most big name grocery stores. Although I don't think Wal-Mart carries it, which is kind of strange.
Anyway, it is a pure form of the ingredient in most "whitening shampoos", plain liquid bluing. Use it diluted in a bit of water and don't leave it on too long or you will have brite blue white spots. I learned this one the hard way just before my championship showmanship class many years ago. I hadn't heard of it and a friend said "here, try it" and didn't warn me. I used it straight from the bottle...

Anybody ever see a palomino with a neon blue tail?? O.O