I like black sand, I use Tahitian Moon Sand, it's black, very fine, heavy (doesn't stay suspended), and inert. most natural fresh water bottoms are dark. In my area the mud is black almost everywhere. In creeks the light sand substrate is usually covered with a layer of diatoms and other films to make it appear dark. I've scuba'd in freshwater and in most areas the substrate is not clean white or tan sand. this sand is often under an inch or so of dark detritus, fish like sunfish will take up residence over these dark bottoms and often display their best colors. Swimming areas (human) are often striped of this detritus layer by the swimming activities of people. Fish tend to hang out at the outer perimeter where they can dash out of the dark zone into the lighter zone to catch food particles.