Flourite "dark" is a dark brown color, and I think it looks good in the tank. As I recall the grain size is a little smaller than regular fluorite, but not nearly as small as flourite sand. If you can find it, see if the color is what you would like.
It is best to assume Flourite of any color is totally inert, so plants don't get any nutrients from it. That means you need to dose the water, and, preferably, use substrate tabs as well as dosing the water.