1Preheat the oven to 325 degrees F. Butter and flour a large tube pan, or use a flour based baking spray such as Baker's Joy. Set the pan aside.
2Separate the eggs, putting the yolks and whites in different mixing bowls. In another large bowl, mix together the flour, baking soda, salt, cinnamon, and nutmeg.
In a small bowl, mix ½ cup of the flour mixture with the walnuts and raisins.
3Add the brown sugar, butter, and blackberry jam to the egg yolks, and stir together thoroughly. Stir in the buttermilk. Add the flour mixture.
Beat the egg whites until they turn white but are loose and runny, not too "airy." Fold the egg whites into the batter. Then mix the nuts and raisins into final mixture blending only enough so that they are equally distributed in the batter.
4Pour the batter into the tube pan and bake for 1 ½ to 2 hours. When a toothpick inserted in the center comes out clean, it's done.
5Remove the pan from the oven and let the cake cool for about 10 minutes in the pan. Then loosen the cake from the sides of the pan by running a knife around the edge. Invert a cake plate over the pan, and invert the two together. Lift off the tube pan.
8 tablespoons butter, softened
1/2 cup heavy cream
2 cups dark brown sugar, packed
Mix all the ingredients together in a saucepan and cook over medium heat until the sugar has dissolved and the icing is well blended. Remove from the heat and drizzle over cake while still warm. You can make the icing as thick as you like.