The microstructure, from the general morphology to the lattice structure, of porous Si layers produced by electrochemical etching of highly doped n-type Si substrates in obscurity conditions was studied by scanning and transmission electron microscopies. Scanning electron microscopy revealed that the porous layer consists of two distinctive layers, characterised by the size of the observed structural features : an upper mesoporous layer and a lower macroporous layer. High resolution electron microscopy showed the presence of thin crystalline Si platelets held at various depths within the pores of the mesoporous layer, in contrast to the Si nanocrystallites and nanowires identified in previous studies. In spite of the different geometry of the nanoparticles resulting from the obscurity conditions, the observed photoluminescence was that of the typical red band.