Etsy has designed blades for ceiling fans that look like bat wings, or dragon wings if bats don't do it for you. They come in sets of 4, 5, and 6 and start at $40. They allegedly move just as much air as less scary looking blades, so you won't lose any efficiency if you buy them. But if you want to be really hardcore, get what I got, which is a razor blade ceiling fan. The first time you take your shirt off while standing underneath is absolutely exhilarating!
Fan Blades For Batman/Dragon Fans [ohgizmo]