Working with Friesians, trying to keep them black is pretty much ALL I do lol.
One is in a sweet itch rug all season long and she's always jet black. But that's also genetic (with Friesians at least, so I'm assuming other black horses too). Some of them just stay jet black regardless more than others. The youngest is bare all year round and she's starting to brown a little bit, but so far it seems minimal. I groom them extensively, but not daily and let the mud/sand do its job.
Now, when it's show season time we do prepare them with colour enhancing shampoos and conditioners. I use a local brand that's marketed specifically for Friesians so I wouldn't know where you would get that from, but my alternative is Gallop's colour enhancing shampoo/conditioner for black horses. After a washing I always top it off with conditioning sprays and a merino wool glove.
And as mentioned above, nutritions.