There are supplements that you can feed, such as Black as Night, keeping him covered, and only turning out at night are my suggestions.
I had a Black TWH that was a "true black" which meant, he never faded. He was turned out 24/7 and never redenned or bronzed. If your horse is not a true black, your only hope is to do the above...
"Be a best friend, tell the truth, and overuse I love you
Go to work, do your best, don't outsmart your common sense
Never let your prayin knees get lazy
And love like crazy"