Toss 4 cups blackberries with the granulated sugar, honey and lemon zest and juice in a medium saucepan. Bring to a boil over medium heat, then reduce the heat to medium low and simmer gently, uncovered, until the berries burst, about 8 minutes. Remove from the heat and stir in the remaining 4 cups berries. Let cool.
Butter the bottom and sides of a 2-quart souffle dish and line with plastic wrap. Trim off the bread crusts, then line the bottom and sides of the dish with a single layer of bread, trimming the slices as needed to cover the dish completely.
Spoon half of the berries and juice into the dish. Cover with a single layer of bread slices. Top with the remaining berries and finish with another layer of bread to completely seal the pudding. Pour any remaining juices over the top layer and cover with plastic wrap. Set a small flat plate on top and weigh it down with a large heavy can. Refrigerate the pudding at least 6 hours or overnight.
Before serving, split the vanilla bean lengthwise and scrape the seeds into a chilled bowl. Add the cream and confectioners' sugar and beat with a mixer until it holds soft peaks.
Remove the plate and plastic wrap and set the souffle dish in a sink filled with 1 inch of warm water to loosen the pudding. Place a serving plate on top, invert the pudding onto the plate and remove the plastic wrap. Serve with the whipped cream and more blackberries.