Here is the flour blend that America's Test Kitchen recommends using in their recipes, which are in the How Can It Be Gluten Free Volume 2. To be fair, when I was eating my blondies today, IF I hadn't made them myself, I would really have wondered. I even thought back and retraced my steps in my mind to make sure I had grabbed the right Tupperware canister. Also if you need Tupperware I know a lady who can get you a great deal ;)
ATK All-Purpose Gluten Free Flour Blend
4 ½ cups plus 1/3 cup white rice flour
1 2/3 cups brown rice flour
1 1/3 cups potato starch
¾ cup tapioca starch
3 tbsp nonfat milk powder
Whisk all ingredients together in large bowl until well combined. Transfer to airtight container and refrigerate for up to 3 months, or freeze for up to 6 months. Bring to room temperature before using.