I know of two things that might help, I used to have a white Palomino and it worked on her pretty well. ESPECIALY the shampoo, she was white after one wash! I LOVE this stuff, but it's a tad expensive, or at least it was at the place where I got it from.
When you bathe her try this shampoo - Gold Nuggets Super Whitening Shampoo
I don't remember the name of this thing, I think it's "Grooming block" or something to that effect. It's a small rectangular black block, when you buy it it will be wrapped in plastic with a maroon label and gold writing and I got it at the Co-op. It will help get the dust off of her.
And "Black as Knight" may make a Whitening product, you would have to look and see. I hope this helps!!
*Edited to add - The block should be called the "Slick'N'Easy"*