Lightly grease your muffin tins. If using paper cups, lightly grease those as well.
Make the blueberry syrup & lemon sugar
In a small saucepan, add 1 cup of blueberries and 1 tsp sugar. Cook over medium heat until reduced to about ¼ of a cup. This should take 5-10 minutes.
While the syrup simmers, zest the lemon and mix with ¼ cup sugar, stirring until the zest is evenly distributed.
In a large bowl, or in the bowl of a stand mixer, beat 8 tbsp butter and 1 cup sugar until combined. (If not using a stand mixer, a hand mixer works too).
Add 2 eggs, one at a time and beat until well incorporated.
Add 2 tsp baking powder, ½ tsp salt, and 1 tsp vanilla to the bowl and beat well.
Gently mix in the flour and milk (over beating at this point will make a denser muffin): Mix in half of the flour (1 cup), then add the milk (½ cup) and stir. Scrape the sides and bottom of the bowl to incorporate any stuff not mixed in. Then add remaining flour (1 cup) and mix until just combined.
Do not add blueberries yet!
Line the bottom of the muffin tins with a little of the blueberry-less batter, about 1 Tbsp. This layer keeps blueberries off the bottom of the muffins and doesn't have to be exact.
Finish assembling the muffins
Once the bottom of the tins are lined with the 'plain' batter, gently fold 1½ cups of blueberries into the batter. No need to toss berries with flour *1 (see note). Fill the muffins tins to about ¾ full (just under ¼ cup of batter).
Swirl in 1 tsp of blueberry syrup into the top of each muffin using a chopstick or straw. Using more than 1 tsp might create a mess.
Lastly, top each muffin with 1 tsp of lemon sugar.
Bake in your 375°F oven for about 30 minutes. Check after 25 minutes since every oven is very different. The top should be golden brown.
When removing from the oven, losen the edges of the muffins from the tin, as the sugar topping can make them stick. Doing this while warm makes it easy, but don't touch any caramelized areas with bare hands, it will burn. Let the muffins cool for 5-10 minutes before removing them entirely from the tins.
1 - If using frozen blueberries, toss them in flour to prevent their juice from dying the batter blue. It does nothing to stop them from sinking, however.