Thoroughly wash the spinach, even if the package says “prewashed.†If using frozen blueberries, quickly rinse them under cold water to thaw a little, and then drain. Fill the bowl of your food processor with the spinach, blueberries, lemon juice, and 1 tablespoon of water; puree on high until as smooth as possible. Stop occasionally to push the contents to the bottom. If necessary, use another tablespoon of water to smooth-out the puree. This recipe makes about 1 cup of puree; double it if you want to store another cup. It will keep in the refrigerator up to 3 days, or you can freeze 1/4-cup portions in sealed plastic bags or small plastic containers.