Oh, I just have to brag. My black boy is in and out as he pleases and not one bleached hair. The secret is to buy a homogygous black. Those that bleach aren't true blacks. My pally, was in during the day and out after dark and since the flies were out she preferred that schedule. This was done only if showing, otherwise she'd fade from fresh minted gold to a bleached blonde.
By this logic a heterozygous black (Bb) isn't true black?
Turning horses out at night doesn't particularly make them more prone to hurting themselves, unless they have some sight defect like moonblindness. They might trip a bit more.
But honestly, at my barn, there is equal turn out at night and day during the summer months and they don't get "watched" any more or less. They might be seen more during the day as people walk back and forth from the pastures more, but they won't be any more or less reckless at night than they would during the day.
My barn owner lets her black TWH out at night only during the summer to prevent his sun bleaching. And he is a black boy, he doesn't turn brown in his coat with sun bleaching, just a lighter black. His mane and tail might bleach out to brown a bit. But it is noticeable when he stays in during the day. She doesn't feed anything extra to keep him black though.
Black oil sunflower seeds gives the coats better shine and health. It also makes sunflowers spontaneously grow in your pasture (great gift by the way haha!).
But as far as I have witnessed, it doesn't particularly affect the color (unless you consider deeper color as a result of better health affecting the actual color). You can add straight oil to their feed and get the same effect. My mother in law feeds BOSS's to her older horses and her goal was to help their hooves. We did see the growth line difference from where she started feeding them BOSS's compared to before they were eating them.