2003 Impala LS

Last month I bought 2003 Impala LS and after couple of weeks of driving I checked the coolant level in the reservoir, it was slightly above the cold line mark ( the engine was cold ). I then opened up the radiator cap and swiped my finger inside the neck of the radiator. I noticed a blackish substance on my finger, I wiped that material off from the radiator neck using a clean cotton cloth. Few weeks later I performed the same test again and observed the same substance deposit at the radiator neck. The coolant level is still at the same level when I last checked. Also the reservoir tank apparently does not seem to contain any impurities ( floating particles etc ), but the color of the coolant does seemed a bit darker than the usual color of dex-cool coolant. My question is what could be this black stuff that I see, is it something normal or am I a proud owner of a gm lemon?.