Preheat the oven to 325 degrees F. Place the bread cubes on a large baking sheet in a single layer and toast until lightly golden brown on all sides, turning a few times, about 15 minutes. Remove and let cool slightly.
Bring cream, 1 cup of sugar, and lemon zest, to a simmer over medium heat. Whisk together the eggs and yolks in a medium bowl. Slowly whisk the warm milk into the eggs and add the vanilla.
Transfer the bread cubes to a 9 by 15-inch baking dish, pour the custard mixture over and press on the bread to submerge in the custard. Let the mixture sit at room temperature for 30 minutes to allow the bread to absorb more of the custard.
Place the baking dish inside a larger roasting pan and fill the outer pan with warm water halfway up the sides of the baking dish. Bake until puffed around the outside but still slightly jiggly in the center, about 45 minutes. Remove from the water bath and let rest at least 15 minutes before serving. Toss the remaining 1/4 cup sugar with the blackberries. Serve bread pudding topped with macerated berries and Meyer Lemon Whipped Cream.
Meyer Lemon Whipped Cream:
Combine the heavy cream, sugar, juice, zest and vanilla extract in a large bowl and whip with a balloon whisk until soft peaks form.