We have a black horse. We tried to keep a sheet on her in the summer so she would stay black, but it was a process. We kept a fly sheet on her and a fly mask with uv protection. Her legs still got bleached and the ends of he mane did too. Our summers are HOT and so she was getting really sweaty even with just the fly sheet, so we stopped with that.
So we got advice from a fellow horse person and they said to let her out at night and keep her inside during the day. We did this for awhile, but we didn't keep on top of it. We let her out all day with the other horses and by the end of the summer, she looked lke a buckskin because she was so bleached out.
Some blacks don't bleach. My friend's black horse is pitch black all year round, no matter how much sun he gets. Unless you are using the horse for showing and want her to look perfect, I wouldn't try too hard to keep her from bleaching out. They will eventuall go back to being black again. There are however, feeds you can give to keep the horses color. We just have never tried them.