• Preheat the oven to 350°F and line a standard loaf pan with parchment paper.

Make the Cake Batter

• In a medium bowl, whisk together the flour, salt and baking powder.
  
• In a large bowl, whisk the eggs and sugar until smooth.
  
• Mix in the yogurt, lemon zest, lemon juice, oil and vanilla into the egg mixture until combined.
  
• Add the dry ingredients to the wet ones and stir just until no dry streaks remain.
  
• Toss the blackberries with a tablespoon of flour and then fold them into the batter gently.
  

Bake the Lemon Blackberry Loaf

• Pour the batter into the prepared loaf pan and smooth the top.
  
• Bake for 50 to 60 minutes, or until a toothpick inserted in the center comes out clean. Try a couple of spots in case your toothpick goes into a blackberry and not the cake itself. Also, if the top browns too quickly, you can loosely cover with foil during the last 10 to 15 minutes.
  
• Let it cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

Prepare the Glaze (Optional)

• Whisk the glaze ingredients together, beginning with 1 tablespoon of lemon juice.
  
• Add more lemon juice a few drops at a time until the glaze is thick but spreadable - it should slowly mound rather than run.
  
• Spread the glaze over the completely cooled loaf using a spatula or spoon.
  
• Let it rest for 15 to 20 minutes before slicing, so the glaze can set.