Spray a Loaf pan with non stick vegetable spray and line the bottom of the pan with parchment paper. Spray the parchment paper with vegetable spray and set aside.
Microwave the blackberry jam in 15-20 second increments until liquidy. Set aside.
In a medium bowl combine the flour, baking powder and salt. Whisk to combine.
In a large bowl beat together the sugar and softened butter for 1 to 2 minutes on medium high speed until fluffy and well incorporated. Beat the eggs in one at a time, scraping the sides of the bowl with a spatula occasionally.
Add 1/3 of the flour to the butter mixture and beat on low speed to combine. Add half of the milk, mixing until just combined, followed by 1/3 of the flour and the other half of the milk. End with the remaining flour mixture. Stir in the meyer lemon zest and set aside.
Add 1/3 of the batter to the prepared loaf pan. Drizzle 1/3 of the blackberry jam over the batter and use a toothpick or skewer to gently swirl the jam into the batter. Top with 1/3 of the cake batter. Gently smooth the batter and drizzle with 1/3 of the blackberry jam. Swirl again with the skewer. Add the final bit of cake batter, smoothing the batter to the edges, followed by the remaining blackberry jam. Swirl again with the skewer.
Bake for 30 minutes, then rotate the pan 180° in the oven and bake for an additional 20-30 minutes or until a cake tester comes out clean. Let the loaf cool for five minutes before removing it from the pan.
Meanwhile, mix the sugar and lemon juice (using the meyer lemon you just zested) together in a small bowl. Spoon it over the still hot loaf and let it come to room temperature before slicing.