I usually take FSW at its word. The people at Grand Prix Models are sticklers for accuracy and have an enviable research library. But there are two problems with the statement about the black Bentley: 1) It is at least half wrong. Duff spent his war on the ground with the Royal Berkshire Regiment. 2) No source is given. I have checked Michael Hay's Bentley: the Vintage Years and Darrell Berthon's A Racing History of the Bentley. Neither mentions a black car. That only leaves a score of Bentley books, a couple of dozen Le Mans histories (in French and English) and a few decades of the Bentley Driver's Club magazine as possible sources. Two of the more likely sources would be Moity and Teissedre's 24 Heures du Mans, 1923-1992 and Dominique Pascal's The British at Le Mans, two books that were frequently cited by FSW around that time. Another possibility would be Elizabeth Nagle's The Other Bentley Boys, which has a chapter on Duff. Unfortunately, I don't have any of them.

Another view: Clement, W.O., Duff and the car in question
  As you can see, the photogaphic evidence isn't much help. Bearing in mind the strange things that old black and white film could do to certain colours, the lighting conditions when the pictures were taken, and the dirtiness of the car, it could have been black. It was certainly darker than Duff's 1923 car. Both cars were private entries. They may have been Duff's personal cars, or whatever the dealership of Duff & Addington could spare.
  The 1924 Le Mans winner still exists and it is painted black, though it seems highly unlikely that it's the original paint. Its owner must have reason to believe that it was black in 1924.
  Does anyone have a source for the black Bentley story?
  Mike