Preheat the oven to 350°F and grease an 8-inch square baking pan.
In a medium mixing bowl, combine the brown sugar, melted butter, egg, vanilla extract, and salt. Stir until the mixture looks smooth and glossy.
Add the flour and baking powder to the bowl. Stir gently until no dry streaks remain, taking care not to overmix the batter.
Fold in the sour cherries gently with a spatula so the fruit stays mostly intact.
Spread the thick batter evenly into the prepared 8-inch square pan.
Bake for 28 to 30 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Allow the blondies to cool completely in the pan before slicing into squares and serving.
