Ahh this is an age old debate! Only recently saw it pop up on another forum too.
If the horse has striped hooves, or any mix of black and white hooves, I black them all. In my opinion, black hooves always look neater and give a "finished" appearance. Striped/odd coloured hooves can also sometimes give the illusion of uneven gaits (same reason dressage riders and hackies prefer horses with even white markings or none at all).
But if they have all white, then sand them back lightly and gloss them. Black hooves on a horse that has naturally all white hooves tends to look out of place because the horse probably has extensive white markings.