Pour watermelon 1 inch of watermelon mixture in molds. Carefully place the blueberrys on top. Slowly add more of the watermelon mixture. Layer in more blueberries. Repeat until molds are full.
Add the top.
Freeze overnight.
To remove run under warm water for 30 seconds.
Notes
I didn't add any extra sugar, but you could add sugar or stevia if you don't find just the watermelon sweet enough!