BLAM notes that Westmalle dubbel has "dark malt for 'aroma'" in addition to caramel malt. It doesn't specify any further but Candy Syrup Inc. has in its clone recipe--using its products of course--Dingemans Mout Roost 900 which is a chocolate malt. Their clone recipes have positive reviews and probably not far off the mark. I'd suspect the recipe has a little more choc malt than what you would use for color adjustment. The Candy Syrup recipe has chocolate malt around 3.5%.