I haven't had it, but their website has enough info for a reasonably educated guess:
Samuel Adams Blackberry Witbier is brewed with two-row Harrington and Metcalfe malts and generous portions of malted wheat.
Flavor: Brewed with Oregon Marion blackberries, orange peel, and coriander to complement the malted wheat.
Color: Hazy gold
Original Gravity: 13.2° Plato
Alcohol by Vol/Wt: 5.5%ABV – 4.3%ABW
Calories/12 oz.: 176
Malt Varieties: Two-row Harrington, Metcalfe and Copeland pale malts, and malted wheat
Hop Variety: Hallertau Mittelfrueh Noble hops
Special Ingredient: Marion blackberries, orange peel, coriander
Yeast Strain: Samuel Adams ale yeast
I'd go with 6 lbs of Wheat DME (which is a blend of wheat and pale barley).
12-15 IBUs of Hallertau (or any European-ish hop at the start of the boil).
.5 oz of corriander @ 5 minutes (I like the sort from an Indian market if you can get it)
.5 oz of freshly grated orange zest @ 0 min
Chill below 70, then pitch a clean ale yeast. White Labs sells their strain:
http://www.whitelabs.com/beer/strains_wlp008.html but any "American" strain will be fine. FG should be ~1.012
For the blackberries I'm not sure if they add extract or real fruit (that fact that it is golden and not purple makes me suspect extract). If you want to go with real fruit, I'd add ~5 lbs of frozen/thawed berries to the secondary for ~2-3 weeks before bottling. For exract, add it to taste in the bottling bucket.
Carbonate to a medium level, ~2.5 volumes.
Hope that helps, good luck.