We reported our detailed investigation of the microstructure and surface chemistry of nanoporous black Si layers using transmission electron microscopy techniques such as HRTEM, EDS, and EELS. We found that a one-step nanoparticle-catalyzed liquid etch creates deep conical nanovoids. The cones provide the density-graded surface that suppresses reflection. The surface of the as-etched nanoporous black Si is an amorphous Si suboxide (SiOx) produced by the strongly oxidizing nanocatalyzed etch. The c-Si/suboxide interface is rough at the nanometer scale and contains a high density of point defects.