Preheat the oven to 350°F(177°C). Grease 8 ½ * 4 ½ inch (21,5*10,5cm) loaf pan and line the bottom and sides with parchment paper, about 2 inches(5 cm) of overhang on each side.
Place the flour (210 grams - 1 and ½ cup), baking powder and salt(optional) in a medium bowl and stir with a whisk. Set aside.
Put the sugar and lemon zest into a large bowl and either rub with your fingers or stir with a fork to bring the lemon flavor out.
Add in the eggs and whisk for almost 20 seconds or until combined well.
Stir in the milk, melted butter, and lemon juice.
Pour the wet ingredients over the flour mixture and stir with a whisk until combined.
Before adding blackberries to the flour mixture you may need to cut them into 2 or 3 pieces: If the blackberries are small, you can use them as they are. If they are medium size, you can cut them into 2 pieces, if they are large, you can cut them into 3 pieces. Do this just before adding them into the batter. Place the blackberries into a medium bowl and add 1 and ½ tablespoons of flour and shake the bowl a couple of times to let flour cover all of them.
Add the floured blackberries into the batter and gently fold with a spoon/spatula.
Spoon the batter into a loaf pan instead of pouring. This way you can see how well blackberries are distributed.
Bake for almost 50-52 minutes or until the toothpick inserted into the center comes out clean.
Let the cake cool in the pan for an hour and then let it cool on a wire rack at least for an hour.