Having Friesian crosses as well as a black TN Walker at one point, I have frequently struggled with keeping their coats black and not fading out in the summer. My TN Walker would have a jet black coat when his summer or winter hair came in, but by the end of the season, he faded to almost a dun color! He was the one horse that seemed to have the most significant color change.
Mane and Tail dye.... Bigen!
Both these horses below are jet black when they first change coats with the seasons. The one on the left (since passed away), would always turn almost a light bay color by the end of summer. My Friesian will turn reddish, as will the tips of his mane and tail, in the sun.