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1
Preheat oven to 350F.
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2
Line bottom of a buttered 9- by 2-inch round cake pan with a round of wax paper, then butter paper.
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3
Sift together flour, baking powder and soda, and salt.
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4
Beat together butter and sugar in a large bowl with an electric mixer until pale and fluffy, then beat in vanilla.
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5
Add eggs 1 at a time, beating well after each addition, then, with mixer at low speed, beat in all of buttermilk until just combined.
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6
Add flour mixture in 3 batches, mixing after each addition until just combined.
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7
Spoon batter into cake pan, smoothing top, and bake in middle of oven until golden and a tester comes out clean, 40 to 50 minutes.
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8
Cool in pan on a rack 10 minutes, then run a thin sharp knife around edge of cake to loosen.
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9
Invert onto rack, then slide cake onto a cake plate.
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10
Bring wine, 3 tablespoons sugar, and zest to a boil in a small heavy saucepan, stirring until sugar is dissolved.
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11
Boil until syrup is reduced to about 2/3 cup, 1 to 2 minutes.
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12
Discard zest.
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13
Reserve 2 tablespoons syrup and pour remainder slowly and evenly over cake (cake will absorb syrup).
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14
Stir together reserved syrup, preserves, and remaining tablespoon sugar in small saucepan and simmer, stirring occasionally, until thickened slightly, about 1 minute (it should be the consistency of a thick syrup).
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15
Put blackberries in a large bowl, then pour preserves mixture over berries.
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16
Gently stir berries with a rubber spatula to coat, then pour over cake, mounding blackberries on top.
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17
Serve warm or at room temperature.
