Preheat oven to 375°F (190°C) and grease an 8x8 inch baking dish.
In a large mixing bowl, combine the rolled oats, baking powder, and sea salt.
In a medium bowl, whisk together the milk, maple syrup, egg, melted butter, vanilla extract, lemon zest, and lemon juice.
Pour the wet ingredients into the dry ingredients and stir until thoroughly combined.
Gently fold in 1 cup of the blueberries.
Transfer the mixture into the prepared baking dish and spread evenly.
Scatter the remaining 1/2 cup of blueberries over the top.
Bake for 35 to 40 minutes, or until the center is set and the top is lightly golden.
Allow the oatmeal to cool for 5 minutes before slicing and serving.