A Bit of History
Since the beginning of cheese making, the preservation of the fresh cheese surface has always been the next major concern after the cheese has left the brine bath or dry salt table. This wonderful rich and aromatic surface has always been just as attractive to the ever-present microbes and mold spores as it has been to us and hence, the race begins. How do we keep the cheese surface in good condition until the cheese has aged enough for the table, whether this was a few days or a few months? Before the invention of the wax or plastic coat and definitely before the present permeable plastic wraps of today, there were far fewer options.
Initially, it was common to just let it go au naturale and accept whatever ambient growth took place, but at times this became a bit too coarse for even the most basic cheese.
Then, at some point long ago, someone had the bright idea of coating the surface with the fine grey ash that was readily available from burnings. This seemed to preserve the cheese by discouraging the flying hoards and the "floaties" from settling and setting up housekeeping on the surface of their cheeses. It also soon became apparent that the ash tended to dry off the surface as well, making it less habitable for the uninvited.
As more time passed, other variations have developed such as the Italian cheese Sottocennere (under the ash) which is buried in a grey ash inside large terra cotta clay pots and aged for months. In France, the Selles sur Cher is another fine example of a young cheese ripened with a layer of ash.
Another example is the Morbier cheese from France with a distinctive black line running through the center of the cheese. The story behind this cheese stems from a time when varying milk sources left cheese makers with half filled molds until the next milking. The problem here was the drying of this surface and the problem with flies and other particulates landing on the surface. The solution was a hand dipped in whey and then rubbed on the surface of the wood fired and heavily carboned vats and transferring this to the surface of the cheese. When the next cheese was made, the new curds were added leaving this distinctive black line through the cheese center.
In the commercial version today this is simply a matter of the traditional appearance. It serves no functional purpose.