the seeds add fat but you'll keep a black, black, by adding 2 tbs of paprika daily to their feed. You would want to start now/soon to effect the spring 2010 coat...you need to be feeding that color when your horse is growing that hair. I just feed it yr round since it's cheap. I get it in bulk from herbalcom.com
If you show it will test for capsacin, fair warning :) so you'd pull them off it 10-14 days before...
It's the main ingredient in "Black as Night (Knight?)", a commercial supplement marketed for dark horses-they have something else for palominos and sorrels.
Costco or a health food store have it in bulk.
Re BOSS, some folks think that the rough edges help grind and kill worms as they grow through the digestive tract-not sure about that but I do like the unexpected volunteer sunflowers in the pasture the next year!