Thats strange I have a registered Black and White Paint and she never fades. We live in the desert as well and she spends most of her time, actually all of it, out in the sun and she never fades. She isnt on any supplements or anything. I think he may be a brown cause true black doesnt fade. At least I don't think.
Black as Knight is a feed through supplement that helps keep dark horses from bleaching out in the sunlight. I'm using it for the first time with my dark bay and so far he's much darker than he was this time last year. I think you have to start it before the sun gets strong. Probably too late to start this year.