I've heard of butter on burns but not on bruises, the best remedy I got that actually works for a while anyway is after you get stung by nettles look around for docking leaves and rub em where u got stung works a treat:)
yeah, if you got e.g. bruise on forehead from walking into a door or something, they used to put butter and cover with a copper coin. Also rub gold on an itchy eye and rub silver on toothaches.
I think it's not so much the fact that you're rubbing butter on a bruise, more the fact that you're rubbing it in the first place. The old saying "Rub it better" definitely works!
Yes, Jaybee, that sounds like a good reason. And isn't it always the way that you bump yourself where it's already painful? (e.g. your bad knee or your 'tennis elbow'!)