in my experience, when the etch depth is in the scale of hundreds of micro
meters, not horizontal scalloping but vertical scalloping can be observed on the
side wall (which can not be found by just inspecting the crosssection). You said
you are doing THROUGH WAFER DRIE, that means there must be another wafer under
the etched wafer to prevent the leakage of helium gas when the wafer is etched
through. In this case, i beleive the black silicon can not be absolutely avoided
but can be alleviated by improving the cooling condition of the silicon wafer
being etched.