I've been doing a lot of googling and trolling of Smartpak as well as other websites looking at coat/hoof supplements. In general I'm not a huge believer in supplements being terribly effective for things like soundness unless they have proven track record. I'd much rather spend my money on injections, which do have a scientific background.
I'm curious but slightly skeptical over things like coat supplements, although they are very tempting for the new owner of a black horse.
A boarder in my barn uses Black As Knight. I've done research on it, and the main ingredient is paprika (a source of copper). So to save money you can just feed paprika. However, it looks like it only works if your horse has a mineral deficiency, and then only when the hair is growing in.
I've heard really good things about Flax seeds, they are 40% fat and are a natural source of omega 3 fatty acids. Adding fat to a horse's diet improves the skin, hair coat, body condition, and can help calm the horse. Flax also has some research reducing sweet itch.
So, if whole flax is a good thing, what about a supplement containing flax plus other stuff? I'm looking at Omega Horseshine. It contains copper and other vitamins to address a mineral deficiency, biotin to address hoof quality, fat to help with shine and weight, and in addition helps with itching caused by allergies, and is supposed to help arthritic horses move better.