The 1941 and 1946 Chevrolet Truck Data Books have a photo for each model. None of the photos show black fenders (as in 1939).
My guess it that black fenders was a very common special order.
Often these special-order paint options were at no added cost (you just had to wait).
However, in 1941, the GMC truck documentation shows that painting any standard color truck with any other standard color fenders resulted in a optional color combination at a cost of $5.10 for most colors and $10.20 for lighter colors (due to bleed-through, no doubt).
