This is a bit more of a personal choice. For me, I love black gravel because it helps bring out the colors in your bettas, fish, snails, shrimp, etc. It also doesn't show all the stuff that can grow on it like tans/whites do. But if you keep up on your cleanings and vacuuming then I would say go for the natural. But it all comes down to personal preference as I would said Black looks simpler.
And I'm not sure what you mean by "dangerous"? I've always used black gravel for my setups for over two years now and nothing's ever happened to my fish or inhabitants, if that's what you're asking :)