I support the previous answer. You are presumably describing minuscule quantities, and in all probability this is skin oils which have become dirty in the pores. The most likely cause of this is atmospheric pollution such as exhaust fumes or general airborne industrial grime. If your working environment exposes you to some of this stuff you will see its presence for sure, as you will if you travel on the London Underground or along most large city streets. To confirm this is what you are looking at, if your tissue/hankie shows very dark mucus when you blow your nose or if cuffs and collars of white clothing turm blach within same day, then you have pollution solids on your skin also.