In a small mixing bowl, mix together 4 tablespoons flour, baking powder, sugar. Cream in butter.
In a separate small mixing bowl, mix milk and vanilla. Add to dry ingredients. Stir to mix.
In a separate bowl, toss blueberries in 1 tablespoon flour. Stir into batter.
Pour the batter into a large (16-ounce or larger) microwave safe mug, lightly sprayed with cooking spray. Microwave the mug on high for 1 minute 30 seconds until thoroughly baked to a firm, not runny texture. Inserted toothpick will come out clean when baked. Eat immediately.