Add the yeast to two cups of warm water (between 100°F and 110°F) in a large bowl. Stir the yeast well and allow it to sit for 5 minutes.
While the yeast is hydrating, zest a large lemon to yield 2 teaspoons of zest. Juice the lemon for 3 tablespoons of juice.
Add the flour, zest, and salt to the yeast and water mixture. Mix well with a wooden spoon or silicone spatula until the flour is evenly hydrated by the water.
Cover the bowl with a kitchen towel and allow to rise for 90 minutes.
Preheat your oven to 425°F. Melt the butter and combine 2 tablespoons of melted butter with the vegetable oil in the bottom of the 9" x 13" baking pan.
Swirl the pan to coat the bottom and about 2-inches up the edges of the pan evenly. Set the remaining 2 tablespoons of butter aside.
Transfer all of the dough to the oiled pan. Spread the blueberries out evenly over the surface of the dough.
Coat your fingers with a bit of extra oil and fold the top third of the dough over. Then fold the bottom third of the dough over the top, completely wrapping the berries in dough (similar to how you'd fold a letter).
Rotate the dough 90 degrees so that the long sides go along the long sides of the pan. Then flip the dough over so that the oiled side is facing up.
Allow the dough to rest in the pan for 20 minutes.
Oil your hands again and stretch the dough to the edges of the pan. Rest the dough for 20 minutes again.
Oil your fingers one last time and dimple the dough, pressing your fingers into the top all over the surface of the dough. Evenly drizzle the dough with the remaining 2 tablespoons of butter.
Bake, uncovered, for 33-35 minutes. The baked focaccia will be browned on top and have golden edges.
Rest the focaccia in the pan for 5 minutes. Transfer it to a wire rack set over a sheet pan to cool for about 20 minutes.
Combine the lemon juice and the powdered sugar with a whisk until completely smooth. Drizzle it over the focaccia evenly.
Allow the focaccia to cool completely before slicing into 16 pieces with a serrated knife.