Preheat the oven to 400ºF.
Line a 1 LB loaf tin with parchment paper, making sure the parchment extends over the sides.
In a large bowl, mix the 2 cups gluten-free flour with the baking powder and coconut sugar.
In a separate bowl, add oat milk, egg, vanilla extract, and lemon juice.
Pour the wet ingredients into the dry ingredients and whisk together.
In a small bowl, mix fresh blueberries and 1 tablespoon gluten-free flour together until blueberries are fully coated with flour.
Pour the floured blueberries into the mixed wet and dry ingredients. It’s fine if loose flour is added into the mix.
Gently fold in the fresh blueberries, until they are fully incorporated into the mix.
Pour the lemon blueberry bread mix into the prepared loaf pan with parchment paper.
Add the blueberry jam on top of the mix in the loaf pan. Place a knife straight down into the bread mix and gently move it around for a marbling effect. Do this until all clumps of jam are spread out and marbled.
Place the cake into the oven in the middle rack and bake for 50-60 minutes or until a toothpick comes out clean. (When doing the toothpick test, be careful. It may come out wet still if you stick it directly through a blueberry.)
Let cool in the loaf pan for 10-minutes.
After 10-minutes, remove the cooked bread from the loaf pan and let cool on a cooling rack for at least 30-minutes.