According to BLAM, Westmalle uses 3 malts in their dubbel. The base malt is a Belgian Pilsen malt. They also use a "local" malt known for its aromatic properties - I'm guessing that this is Belgian Aromatic. They also use a darker, sweeter malt. I'm not sure what this is - I'm going to experiment with Belgian Cara 45L, Belgian Caramunich, and Special B. Hopefully, it's something like that.
Westmalle also mixes up their hop schedule from year to year. I'm thinking a schedule consisting of Styrian Goldings, Tettnang, and Saaz will get you in the ballpark.
The dark color, and dark fruit aroma and flavor comes from a dark candi syrup that they use. I'm going to try D2 when I attempt my clone.
And of course, you'll want to use Westmalle's yeast strain, Wyeast 3787 Trappist High Gravity yeast.