Place your watermelon (seeds excluded) into a blender and blend into juice. You can alternatively use a juicer for a smoother final product depending on your preference.
Make a simple syrup by combining the sugar, water, and black sesame seeds in a small saucepan and bringing to a boil, stirring just until the sugar dissolves. Take off the heat and add the toasted sesame oil. Let the simple syrup cool to room temperature.
Combine the watermelon juice and simple syrup with a pinch of salt and freeze in popsicle molds, about 4 hours to completely set. Enjoy!