Category 3 All Black - Black Tungsten Bead

Turn over a rock and see how much of it is black

Most of what lives on a river bed is small and dark, and a black nymph covers more of it than any single imitative pattern will. That is the whole case for carrying one: not because it matches a particular insect but because it matches the general run of them. Fish it as the dropper when you have no information, and let something heavier below it do the depth work.