Bill, my guess would be that the WD-40 mixed with the carbon black was actually penetrating the older faded and patina black oxides and dyeing them. It might also darken bare steel as layout blue does, but this would wear off quickly. This would be much the same as using logwood chips to dye darken barrels which have been browned to increase the contrast.
I don't know what it is about WD-40, but it does penetrate and loosen heavy patina when used with 0000 steel wool better than other lubricants. It will also remove some of the oxides on freshly browned or blacked barrels, so it should not be used to prevent after rusting after neutralizing and rinsing.
