Beat margarine until creamy and fluffly. Gradually, add sugar and eggs, one at a time.
Remove from mixer and sift in flour, blackberries and lemon juice.
Transfer to a greased and floured pan 11 x 5.5 x 3 inches (22 x 11 x 6 cm).
Bake in pre-heated oven 350° F (175° C) for 30 to 35 minutes or until the blade of a thin knife, when inserted, comes out clean and dry.
Cool and unmold. Dust confectioners sugar on top.