Yeah this is kind of a testy subject for a lot of ray keepers, because there are die hards on both sides. Personally, I keep most of my rays on bare bottom because, to Chen's point, rays are filthy animals and they make a hell of a mess. Bare bottom is just cleaner and easier to maintain.
However, I do think rays (especially black rays) look better on black sand. This isn't just my opinion, I have seen first hand with my hen's how much brighter their spots and darker their base looks on black sand vs. barebottom. Also, with sand you get to see all kinds of cool behaviour that you would otherwise miss out on with a barebottom.