Puree watermelon, lime juice and sugar to taste in a food processor or blender until smooth.
Divide blueberries among freezer-pop molds. Top with the watermelon mixture. Insert the sticks and freeze until completely firm, about 6 hours. Dip the molds briefly in hot water before unmolding.
Notes
Make Ahead Tip: Freeze for up to 3 weeks. | Equipment: Ten 3-ounce (or similar-size) freezer-pop molds