Preheat the oven to 350 degrees. Line a 9x13 baking dish with parchment paper allowing for overhang so that you can lift the cooled bars out of the pan to cut.
In a standing mixer, add the flour, sugar, and lemon zest. On low speed, mix until combined.
Add in the butter and mix until you get pea-sized pieces of butter.Tip: This can also be done in a food processor, or in a large bowl with a pastry cutter.
Add in the beaten eggs and mix until a crumbly dough comes together.
Press 2/3 of the dough evenly into the bottom of the parchment-lined baking dish and set aside.
In a large mixing bowl combine the blackberries, lemon juice, cornstarch, and sugar. Mix until combined.
Pour the blackberry mixture over the crust and even it out with a rubber spatula.
Crumble the remaining dough over the blackberry filling.
Bake in the preheated oven for 45 to 50 minutes or until the crumble appears slightly golden brown.
Pull the baking dish out of the oven and set it aside to cool completely before cutting bars.