Combine the blueberries, sugar, and lemon juice in a medium saucepan. Bring to a boil, then reduce the heat to medium and simmer for about 15 minutes, stirring occasionally until thickened and some blueberries are mashed. refrigerate the preserves while the dough rises.

In a mixing bowl, beat together cream cheese and sugar until smooth. Set aside for later use.

In a large bowl, whisk together flour, ⅓ cup sugar, and salt. Heat the milk in the microwave until it reaches about 100–110°F. Pour the milk into the bowl of a stand mixer, sprinkle yeast over the milk, and stir in 1 teaspoon sugar. Let the mixture sit for about 5 minutes to allow the yeast to activate.

Add eggs, vanilla, flour, sugar, and salt to the yeast mixture. Mix on low speed until the dough comes together, then increase to medium-low speed and knead for about 5 minutes until the dough is smooth.

Gradually add the softened butter, 1 tablespoon at a time, mixing on medium-low until incorporated and the dough is sticky. Scrape the sides as needed.

Cover the dough with plastic wrap and let it rise in a warm place for 1 ½ hours, or until it doubles in size. (Alternatively, refrigerate overnight for a more flavorful dough.)

Punch down the dough, scrape it out onto a floured surface, and divide it in half. Roll each portion into a 12 x 16 inch rectangle. Spread half the cream cheese mixture on each, leaving a small border. Add half the blueberry preserves on top of the cream cheese layer.

Starting from the edge without the border, roll the dough tightly into a log. Trim about 1 inch from each side. Slice the log down the center lengthwise to expose the layers. Twist the two halves over each other, pressing lightly as you go to form a braid. Repeat with the other half of the dough.

Line two 9 x 5 loaf pans with parchment paper. Gently place each braided dough into the pans, pressing the ends slightly to fit. Cover with plastic wrap and let rise for an hour.

Preheat the oven to 350°F (175°C). Bake the babkas for 40 minutes, or until golden brown and a tester comes out clean.

While the babkas bake, prepare the sugar syrup by heating the sugar and water in a saucepan over medium-high heat until dissolved. Brush the sugar syrup over the babkas immediately after baking, using it all.

Allow the babkas to cool for a few minutes in the pans, then transfer to a wire rack to cool completely.