Preheat oven to 350°F. Grease or parchment line and grease an 8x8" or 9x9" square baking pan.
Beat the butter and sugar with an electric mixer at medium speed for 2-3 minutes until light and fluffy.
Scrape down the sides of the bowl. Then, with the mixer running at low speed add the egg, vanilla, salt, and lemon zest. Increase speed to medium-high and beat an additional 2-3 minutes until glossy, airy, and smooth. Scrape down the sides of the bowl as needed, and when finished.
Add flour all at once, then mix the batter at low speed, just until combined.
In a small bowl, toss berries with 1 teaspoon sugar, and juice from ½ a lemon. Mash with a fork or muddler to help the berries break down. (If you want whole berries or larger berry bits, don't mash them too much.)
Add the berry mixture to the blondie batter. For a marbled effect, stir minimally with a spatula. For a more consistently pink-colored blondie, mix until no streaks remain.
Pour batter into prepared baking pan and bake for 20-25 minutes. Check at 20 minutes — blondies are better slightly under baked than over baked.The center should still be a bit jiggly when you take them out of the oven; they will continue to bake as they cool.
Remove blondies from oven and cool completely in the pan on a wire rack before slicing and serving.