1) Preheat your oven to 350°F. Lightly grease a 10" tube pan; a 9" x 13" baking pan, or two 8" x 2" round pans.
2) To make the lemon streusel: In a medium-sized bowl whisk together the sugar, flour, and salt. Gently toss the candied peel until it is well coated. If you prefer smaller pieces of peel, you can pulse the mixture 10-12 times with a food processor at this point. For a bigger lemon "bite" leave as is.
3) Work the softened butter into the flour/peel mixture until it resembles granola, some larger chunks, some smaller. Sprinkle on the lemon oil and toss again to combine. Set the mixture aside.
4) To make the cake: Beat the butter and sugar until light colored and fluffy. Add the eggs one at a time and beat after each addition. Be sure to scrape down the bowl well each time. Beat in the sour cream and vanilla.
5) In a separate bowl, whisk together the flour, baking powder, baking soda and salt. Add to the batter, mixing until combined. Avoid over-beating or the cake will become tough.
6) Gently fold in the blueberries until well distributed throughout the batter. Pour into the prepared pan(s); sprinkle the lemon streusel evenly on top.
7) Bake the cake for about 40 minutes (in the tube pan), or 30 to 35 minutes in the 9" x 13" or cake pans, or until a toothpick inserted into the center of the cake comes out clean. Remove from the oven, and cool on a rack for 15 to 20 minutes before serving.
Yield: about 2 dozen servings.