The only thing I can think that will work is putting her in during the day and out at night . A few times blanketing isn't going to stop the fading in the sunlight. I have a black horse and in the summer time he is just a black horse with highlights !
I have found nothing that works to keep my black horse black. She lives outside 24/7, wears a fly sheet in summer and blankets in winter, used to give her paprika- she hated it and I saw no change. I use shampoo for black horses.
OUr black twh gets bleached out bad. We started giving ground flax in july of this year and within 6 weeks he black as night. Now I had to trim the sunfaded parts off his mane and tail but his coat looked fantastic.
I learned sweat is your enemy when trying to keep your black horses black. Wherever they sweat they tend to bleach out quicker so take the time to rinse it off. Both my black but not really black horses are outside and do wear blankets.
After having her, I now have Zena who is the identical color even down to the rabicano roaning. In the beginning I didn't really worry too much about the sweat but when she began bleaching I took the time to rinse her off after riding her and now she stays very black.