Its a little labor intensive, but I keep the sweat rinsed off of Vida and that seems to help. I also use Vitrolin Shine which contains a sunscreen on her.
She fades more from sweat than from the sun. If I don't rinse her off after a ride she will be a reddish color just where the saddle sits and the girth line. Pretty weird looking. I rinse her off every time I ride and whenever its really hot and she is sweaty. Then spray more Vitrolin on her. Short of keeping her covered, inside during the day or washing her in a darkening shampoo, that's all I can do.
She is considered a black horse just what is called a "faded black"
Quote from www.ultimatehorsesite.com
There aren't really 'shades' of black, but there are two types: fading and nonfading. Nonfading black is also called raven or jet black. It does not fade, or it fades only under harsh conditions. Jet black horses are usually born charcoal or smokey colored, but rarely are black at birth. Nonfading horses have a metallic, bluish, or iridescent sheen and are born a bluish-black, charcoal, or black color.
Fading black horses get reddish-tinged hairs or brown burned areas from sweat or sun. They may also fade seasonally or with poor nutrition. To bring out their best color, fading blacks must be kept out of the sun or blanketed so that their hairs are not bleached. These horses are still blacks, even though during certain times they may look dark brown. They are born a smoky color, or sometimes dark bay or brown.
Some horses may not quite fit either description or may be tough to categorize-- such a horse can safely be called just "black".