In a small saucepan, combine ginger, 1/2 cup of the sugar, and 1/2 cup water. Bring to a boil, and stir until sugar dissolves. Cover pan and remove from heat; set aside to steep, at least 20 minutes and up to 1 hour. (The longer the syrup steeps, the stronger the ginger flavor will be.)
With a serrated knife, cut pound cake into 1-inch-thick slices; lay slices on a piece of waxed paper (for easier cleanup). With a brush, dab slices on both sides with syrup, leaving chopped ginger behind; use all the syrup. Cut slices into cubes.
Whip cream with remaining 1/4 cup sugar until soft peaks form. In alternating layers, arrange cake cubes, whipped cream, and blackberries in 8 serving glasses or dishes (or one large, deep serving dish, preferably glass). Chill until ready to serve, at least 30 minutes and up to overnight.