A perfect clean patina on iron will remain brown and not turn black. If you were to heat oil and cool and wipe and do this a hundred times, I would expect a dark brown color on a iron Debuyer. The black is just small amounts of microscopic toasted foody carbonized goodness. My three Mineral B pans still remain a mottled brown because I am anal about wiping them down after use. They are slick as heck.
On a side note--- do people here regularly season the bottom side of their pain because they get sick of the patina/rust? My bar mops start taking on a nice rusty color when I wipe the stove side of my Debuyers. Heh.