Preheat your oven to 400 degrees and grease 1-2 muffin tins or line with cupcake liners.
In a mixing bowl, whisk together the flour, baking powder, and salt. Set aside.
In another mixing bowl, cream together the butter and sugar with a hand mixer (or using a stand mixer) until fluffy.
Beat in the egg, vanilla, yogurt, lemon zest, and lemon juice.
Add the dry ingredients in about 4 increments until combined, scraping down the sides of the bowl as needed.
Turn off your mixer and fold in the blackberries until just combined.
In a separate, small bowl, use a fork to mix together the melted butter, sugar, flour, and salt for the crumb mixture.
Fill the wells of the prepared muffin tin around ⅔ full with the batter.
Spoon a tablespoon or two of the crumb mixture over the batter in each well of the muffin tins.
Place your muffin tins intro the oven and bake for 5-6 minutes, then reduce the temperature to 350 degrees.
Bake for an additional 16-18 minutes or until the tops have risen and a toothpick or cake tester comes out clean when inserted into the center of the muffins. Allow them to cool.
Meanwhile, use a fork or whisk to combine the powdered sugar and lemon juice for the glaze.
Spoon a little glaze over each muffin, then serve.