Could she be really sunburnt? Perhaps copper deficient? She certainly looks like she should still be black...
Maybe get a good supplement into her, to raise her copper levels, and start putting a fly sheet on her, to protect her from the sun's rays...see if that doesn't bring out her true coloring back.
My last horse was a very dark colored boy, and when he started developing a really burnt coat, especially at the top of his tail head, and top of his mane, I had him tested for copper deficiency; he was pretty low, so I had to get him on a copper supplement, and even when things leveled out, I was told that due to his dark coloring, to make sure I always made sure to keep a mineral supplement with adequate copper in it available at all times...
Pride the first summer I had him...
The following summer...
And not too long before he passed...
As you can see from the pictures, it can take a while for the true color to come back, especially if there is a really low deficiency...