From Brew Your Own... http://www.byo.com/stories/recipeindex/article/recipes/108-porter/1843-blackberry-porter
5 gallons; O.G. = 1.045 (plus a bit)
* 6 lbs. Amber malt extract syrup
* 1 lb. British caramel malt
* 1/2 lb. Chocolate malt
* 5 lbs. Crushed blackberries
* 1.5 oz. Northern Brewer hop pellets
In this case the crushed caramel and chocolate malts are steeped for one hour in one gallon of 150°F water. Then they are rinsed with hot water (about 170°F) and the liquid collected. Enough water is added to bring the volume to 2.5 gallons and the whole shebang is boiled for 90 minutes. The hops are added at the 15 minute mark.
The hot wort is poured over the blackberries in an open fermenter, then 2.5 gallons of pre-boiled, cooled water is added. The blackberries are held in a coarse mesh bag designed for making wines. While not absolutely necessary, it does help keep down the mess. As always, the wort is vigorously aerated and a starter culture of good ale yeast is pitched.
Everything else follows normal fermentation and bottling procedures. The open fermenter is really essential for this process, using whole fruit. After the primary fermentation is completed, the beer is racked into a carboy for clearing and then bottled with 3/4 cup corn sugar.
That seems really solid for letting the blackberries shine. By going with a more of a brown porter recipe, the flavors of caramel and chocolate will still be there, but the fruit notes from the blackberries should be able to come through nicely. Note that it is longer steep and boil.
YMMV of course. Sounds awesome to me.