Looks like rubber paint to seal the cone against water ingress/beer splashes (I worked in the night club sound and lighting game years ago), we did the same to Bose 101's facing the sea on a bar wall, extended the life from ~18 months to ~5 years. Most speakers only change response slightly due to the extra weight to move, though many speakers are now made with plastic cones for such use, I see polished Aluminium cones are now popular for high power systems where paper, even rubber painted, cannot cope with the high power large movements required.