This is a great recipe. I used only frozen blackberries and it turned out perfectly. I wasn't so sure about adding the juice to the filling as it turned an unappetizing color. The taste, however, changed my mind. Delicious! To make it more beautiful :) I might experiment with adding more berries and more juice.
Couldn't find mulberries fresh, frozen or dried around here, so substituted 1/4 cup of frozen mixed berries for them & that worked just fine! I used unsalted butter & whole wheat flour in the crust & then followed the recipe on down (well, with this exception ~ I added a rounded teaspoon of lemon zest to the filling)! These are GREAT, GREAT LEMON BARS & a nice change from those I usually make! Thanks for sharing! [Tagged, made & reviewed for one of my adopted chefs in the current Pick A Chef]