I actually used to use bluing on my horses socks when I bathed her, and it made them insanely white! (Bluing used to be used for laundry to clean white clothes and give them a very slight blue tint to make them look extra white). It is totally safe to use, at least the variety I had was. It never once irritated my horse (but if you use just make sure it doesn't, just in case). I used the tiniest amount and diluted it in water, then dipped a small brush in it and brushed it over her socks. Then I let it sit for a couple minutes then rinsed it off with the rest of her. Careful not to use too much though or it will definitely make their socks blue (which, although cool, might not be good for a show).
I dunno if this picture does it justice for how white her socks were at this show, but they do still look pretty white! https://fbcdn-sphotos-e-a.akamaihd.n...147_8232_n.jpg