Cut the cake into slices and arrange them on a plate or a wide, shallow dish. Drizzle with the orange liqueur.
Zest the orange or clementines into a bowl and set aside. Squeeze the juice from the orange or clementines, over the liqueur-soaked cake.
Whip the cream in a small bowl until thick but soft, and spoon it unfancily over the top of the saturated, not to say gloriously sodden, cake.
Arrange the blackberries over the top of the whipped cream, then scatter with the reserved zest before serving.