Heat ovewn to 350. Spray an 9x5 loaf pan well. If you have parchment paper, spray then line pan with parchment paper, leaving a couple of inches overhanging on all sides, makes for even easier removal! In a blender or food processor puree blackberries with 2 tablespoons of sugar. In a medium bowl, whisk together flour, salt and baking powder.
In a large bowl, using an electric mixer beat together butter and 1 1/4 cups sugar until fluffy (about 5 mins.). Add eggs and vanilla and beat until combined. With mixer on low, add flour mixture in 3 additions, alternating with sour cream. You should begin and end with flour mixture.
Pour half the batter to pan and dot with 1/2 cup of blackberry puree. Repeat with remaining batter and puree. Using a skewer or butter knife, swirl batter. Bake for about an hour or until golden brown and toothpick test comes out clean. Let cool for about 30 mins. and remove from pan.