Dissolve soda in buttermilk, stirring well; set aside. Combine flour, baking powder, salt, cocoa, cinnamon, and allspice; add to the creamed mixture alternately with the buttermilk mixture, beginning and ending with flour mixture. Fold in blackberry jam.
Pour batter into a greased and floured 10 - inch Bundt pan. Bake at 350° for 45 to 50 minutes or until cake tests done. Cool in pan 15 minutes; remove from pan, and cool completely. Spoon glaze over top of cake.