Preheat the oven to 350° F. Prep a 13x9 baking dish or two 9-inch diameter cake pan by greasing it with butter and lightly flouring the bottom and sides.
In a small bowl, stir together the chocolate and hot water until the chocolate is melted. Set aside to cool.
In a medium bowl, whisk together the flour, cornstarch, cocoa powder, baking soda and salt. Set aside.
In the bowl of a stand mixer, cream together the butter and sugar until light fluffy. Add eggs one at a time, mixing well after each addition. Mix in the vanilla extract.
Turn the mixer to low. In alternating steps, slowly add the flour mixture and then the milk, repeat and end with the flour mixture. Mix just until combined. Slowly add the cooled chocolate mixture. Mix until smooth. Fold in the blueberries.
Pour the batter into the prepared pan(s) and bake for 45 to 55 minutes, or until a toothpick inserted comes out clean or with just a couple crumbs. Let the cake cool in the pan and serve at room temperature with the blueberry sauce.
Combine the blueberries and sugar in a large heavy saucepan. Add 1/4 cup water and the lemon juice and bring to a boil over high heat, stirring occasionally. Reduce the heat and simmer for 15 minutes, or until the mixture begins to thicken.
Remove from the heat. Cool to room temperature, cover and refrigerate. When ready to serve, slice a piece of cake and drizzle with blueberry sauce.
*If you choose to use frozen blueberries in the cake batter, toss them in flour before adding them to the batter. This will keep them from sinking to the bottom of the cake.