Preheat oven to 350 degrees F. Spray bottom and sides of a 9" x 5" loaf pan with cooking spray (or grease with butter) and set aside. Rinse blueberries and pat dry as best you can. Toss blueberries in a bowl with 1 tbsp of all-purpose flour until they are all coated evenly. Set aside.
In a large bowl or the bowl of a stand mixer, cream butter. Add 1 cup granulated sugar and continue to mix. Add eggs, vanilla extract, baking powder, and salt and mix in until mixture is smooth.
Add the flour and buttermilk in two increments while the mixer is on low speed: add half of the flour (1 cup), and then half of the buttermilk (1/4 cup). Repeat with remaining flour and buttermilk and mix until just combined and there are no large lumps remaining. Fold in blueberries.
Pour batter into prepared loaf pan. In a medium bowl, add all of the streusel ingredients and mix using a wooden spoon or a whisk so that there are no lumps of brown sugar remaining and the streusel is a semi-moist consistency that can clump easily. Scatter streusel evenly over top of the batter in the loaf pan.
Bake for 45-50 minutes, or until the top has turned a nice golden brown color and a long, thin tester inserted into the center of the loaf comes out clean. If the top of the bread has browned before the middle is finished baking, place an aluminum foil "tent" over top of the loaf and continue to bake until the middle is done.
Once done, remove pan from the oven and place the bread still in the pan on a wire rack to cool for at least 10-15 minutes. Use a knife to loosen the edges of the bread and carefully invert to remove to a wire rack to cool completely. Cut into slices once cooled (earlier could cause exceptionally crumbly slices).
To make buttermilk on the fly, I added 1/2 tbsp white distilled vinegar to 1/2 cup 1% milk and let the mixture sit for 5-10 minutes.I used an uncooked piece of spaghetti as my long, thin tester!Adapted from Food Network